ML Aggarwal Solutions for Class 10 Maths Chapter 9 Arithmetic and Geometric Progression are provided here. BYJUâ€™S tutor team has prepared solutions, which help students attain good marks in Maths. From the exam point of view, the solutions are solved in a simpler manner where students can secure an excellent score by solving the ML Aggarwal textbook. Solutions that are provided here will help you in getting acquainted with a wide variety of questions and thus, develop problem-solving skills. Download pdf of ML Aggarwal Solutions for Class 10 Maths Chapter 9 in their respective links

Chapter 9 – Arithmetic and Geometric Progression. Arithmetic progression is a sequence of numbers such that the difference between the consecutive terms is constant. Difference here means the second minus the first. Geometric progression is defined as a sequence of numbers, where each term after the first is found by multiplying the previous one by a fixed, non-zero number called the common ratio. In these ML Aggarwal Solutions for Class 10 Maths Chapter 9, students are going to learn solving different types of problems and topics of Arithmetic and Geometric Progression.

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Exercise 9.1

**1. For the following A.P.s, write the first term â€˜aâ€™ and the common difference â€˜dâ€™:**

**(i) 3, 1, â€“ 1, â€“ 3, â€¦**

**Solution:-**

From the question,

The first term a = 3

Then, difference d = 1 â€“ 3 = – 2

– 1 â€“ 1 = – 2

– 3 â€“ (-1) = – 3 + 1 = -2

Therefore, common difference d = – 2

**(ii) 1/3, 5/3, 9/3, 13/3, â€¦.**

**Solution:-**

From the question,

The first term a = 1/3

Then, difference d = 5/3 â€“ 1/3 = (5 – 1)/3 = 4/3

9/3 â€“ 5/3 = (9 – 5)/3 = 4/3

13/3 â€“ 9/3 = (13 – 9)/3 = 4/3

Therefore, common difference d = 4/3

**(iii) -3.2, -3, -2.8, -2.6, â€¦.**

**Solution:-**

From the question,

The first term a = -3.2

Then, difference d = -3 â€“ (-3.2) = -3 + 3.2 = 0.2

-2.8 â€“ (-3) = -2.8 + 3 = 0.2

-2.6 â€“ (-2.8) = -2.6 + 2.8 = 0.2

Therefore, common difference d = 0.2

**2. Write first four terms of the A.P., when the first term a and the common difference d are given as follows:**

**(i) a = 10, d = 10**

**Solution:-**

From the question it is given that,

First term a = 10

Common difference d = 10

Then the first four terms are = 10 + 10 = 20

20 + 10 = 30

30 + 10 = 40

Therefore, first four terms are 10, 20, 30 and 40.

**(ii) a = -2, d = 0**

**Solution:-**

From the question it is given that,

First term a = -2

Common difference d = 0

Then the first four terms are = -2 + 0 = -2

-2 + 0 = -2

-2 + 0 = -2

Therefore, first four terms are -2, -2, -2 and -2.

**(iii) a = 4, d = -3**

**Solution:-**

From the question it is given that,

First term a = 4

Common difference d = -3

Then the first four terms are = 4 + (-3) = 4 – 3 = 1

1 + (-3) = 1 â€“ 3 = – 2

-2 + (-3) = -2 â€“ 3 = – 5

Therefore, first four terms are 4, 1, -2 and -5.

**(iv) a = Â½, d = -1/6**

** Solution:-**

From the question it is given that,

First term a = Â½

Common difference d = -1/6

Then the first four terms are = Â½ + (-1/6) = Â½ – 1/6 = (3 – 1)/6 = 2/6 = 1/3

1/3 + (-1/6) = 1/3 â€“ 1/6 = (2 – 1)/6 = 1/6

1/6 + (-1/6) = 1/6 â€“ 1/6 = 0

Therefore, first four terms are Â½, 1/3, 1/6 and 0.

**3. Which of the following lists of numbers form an A.P.? If they form an A.P., find the common difference d and write the next three terms:**

**(i) 4, 10, 16, 22, â€¦**

**Solution:-**

From the question it is given that,

First term a = 4

Then, difference d = 10 â€“ 4 = 6

16 â€“ 10 = 6

22 â€“ 16 = 6

Therefore, common difference d = 6

Hence, the numbers are form A.P.

**(ii) -2, 2, -2, 2, â€¦**

**Solution:-**

From the question it is given that,

First term a = -2

Then, difference d = -2 â€“ 2 = – 4

-2 â€“ 2 = -4

2 â€“ (-2) = 2 + 2 = 4

Therefore, common difference d is not same in the given numbers.

Hence, the numbers are not form A.P.

**(iii) 2, 4, 8, 16, â€¦**

**Solution:-**

From the question it is given that,

First term a = 2

Then, difference d = 4 â€“ 2 = 2

8 â€“ 4 = 4

16 – 8 = 8

Therefore, common difference d is not same in the given numbers.

Hence, the numbers are not form A.P.

**(iv) 2, 5/2, 3, 7/2, â€¦**

**Solution:-**

From the question it is given that,

First term a = 2

Then, difference d = 5/2 â€“ 2 = (5 – 4)/2 = Â½

3 â€“ 5/2 = (6 – 5)/2 = Â½

7/2 â€“ 3 = (7 – 6)/2 = Â½

Therefore, common difference d = Â½

Hence, the numbers are form A.P.

**(v) â€“ 10, -6, -2, 2, â€¦**

**Solution:-**

From the question it is given that,

First term a = -10

Then, difference d = -6 â€“ (- 10) = – 6 + 10 = 4

-2 â€“ (-6) = – 2 + 6 = 4

2 â€“ (-2) = 2 + 2 = 4

Therefore, common difference d = 4

Hence, the numbers are form A.P.

**(vi) 1 ^{2}, 3^{2}, 5^{2}, 7^{2}, â€¦**

**Solution:-**

From the question it is given that,

First term a = 1^{2} = 1

Then, difference d = 3^{2} â€“ 1^{2} = 9 – 1 = 8

5^{2} â€“ 3^{2} = 25 – 9 = 16

7^{2} â€“ 5^{2} = 49 – 25 = 24

Therefore, common difference d is not same in the given numbers.

Hence, the numbers are not form A.P.

Exercise 9.2

**1. Find the A.P. whose n ^{th} term is 7 â€“ 3K. Also find the 20th term.**

**Solution:-**

From the question it is given that,

n^{th} term is 7 â€“ 3k

So, T_{n} = 7 â€“ 3n

Now, we start giving values, 1, 2, 3, â€¦ in the place of n, we get,

T_{1} = 7 â€“ (3 Ã— 1) = 7 â€“ 3 = 4

T_{2} = 7 â€“ (3 Ã— 2) = 7 â€“ 6 = 1

T_{3} = 7 â€“ (3 Ã— 3) = 7 â€“ 9 = -2

T_{4} = 7 â€“ (3 Ã— 4) = 7 â€“ 12 = – 5

T_{20} = 7 â€“ (3 Ã— 20) = 7 â€“ 60 = – 53

Therefore, A.P. is 4, 1, -2, -5, â€¦

So, 20^{th} term is â€“ 53

**2. Find the indicated terms in each of following A.P.s:**

**(i) 1, 6, 11, 16, â€¦; a _{20}**

**Solution:-**

From the question,

The first term a = 1

Then, difference d = 6 â€“ 1 = 5

11 â€“ 6 = 5

16 â€“ 11 = 5

Therefore, common difference d = 5

From the formula, a_{n} = a + (n – 1)d

So, a_{20} = a + (20 – 1)d

= 1 + (20 – 1)5

= 1 + (19)5

= 1 + 95

= 96

Therefore, a_{20} = 96

**(ii) -4, -7, -10, -13, â€¦, a _{25}, a_{n}**

**Solution:-**

From the question,

The first term a = -4

Then, difference d = -7 â€“ (-4) = – 7 + 4 = -3

-10 â€“ (-7) = -10 + 7 = -3

-13 â€“ (-10) = -13 + 10 = -3

Therefore, common difference d = -3

From the formula, a_{n} = a + (n – 1)d

So, a_{25} = a + (25 – 1)d

= -4 + (25 – 1)(-3)

= -4 + (24)-3

= – 4 – 72

= -76

Therefore, a_{25} = -76

Now, a_{n} = a + (n – 1)d

a_{n} = -4 + (n – 1)-3

= -4 â€“ 3n + 3

= -1 â€“ 3n

**3. Find the n ^{th} term and the 12^{th} term of the list of numbers: 5, 2, â€“ 1, â€“ 4, â€¦**

**Solution:-**

From the question,

The first term a = 5

Then, difference d = 2 â€“ 5 = – 3

-1 â€“ 3 = -3

– 4 â€“ (-1) = -4 + 1 = -3

Therefore, common difference d = -3

From the formula, a_{n} = a + (n – 1)d

T_{n} = a + (n – 1)d

= 5 + (n – 1)-3

= 5 â€“ 3n + 3

= 8 â€“ 3n

So, T_{12} = a + (12 – 1)d

= 5 + (12 – 1)(-3)

= 5 + (11)-3

= 5 – 33

= – 28

**4. (i) If the common difference of an A.P. is â€“ 3 and the 18 ^{th} term is â€“ 5, then find its first term.**

**Solution:-**

From the question it is given that,

The 18^{th} term = -5

Then, common difference d = -3

T_{n} = a + (n – 1)d

So, T_{18} = a + (18 – 1)d

-5 = a + (18 – 1)(-3)

-5 = a + (17)(-3)

-5 = a – 51

a = 51 â€“ 5

a = 46

Therefore, first term a = 46

(ii) If the first term of an A.P. is â€“ 18 and its 10th term is zero, then find its common difference.

**Solution:-**

From the question it is given that,

The 10^{th} term = 0

Then, first term a = -18

T_{n} = a + (n – 1)d

So, T_{10} = a + (10 – 1)d

0 = -18 + (10 – 1)d

0 = -18 + 9d

9d = 18

d = 18/9

d = 2

Therefore, common difference d = 2

**5. Which term of the A.P.**

**(i) 3, 8, 13, 18, â€¦ is 78?**

**Solution:-**

Let us assume 78 as n^{th} term.

From the question,

The first term a = 3

Then, difference d = 8 â€“ 3 = 5

13 â€“ 8 = 5

18 â€“ 13 = 5

Therefore, common difference d = 5

T_{n} = a + (n – 1)d

So, 78 = a + (n – 1)d

78 = 3 + (n – 1)5

78 = 3 + 5n – 5

78= -2 + 5n

5n = 78 + 2

5n = 80

n = 80/5

n = 16

Therefore, 78 is 16^{th} term.

**(ii) 18, 15Â½, 13, â€¦ is â€“ 47?**

**Solution:-**

Convert mixed fraction into improper fraction = 15Â½ = 31/2

Let us assume -47 as n^{th} term.

From the question,

The first term a = 18

Then, difference d = 31/2 â€“ 18 = (31 â€“ 36)/2 = -5/2

13 â€“ 31/2 = (26 – 31)/2 = -5/2

Therefore, common difference d = -5/2

T_{n} = a + (n – 1)d

So, -47 = a + (n – 1)d

-47 = 18 + (n – 1) (-5/2)

-47 = 18 â€“ 5/2n + 5/2

-47 â€“ 18 = -5/2n + 5/2

-65 = -5/2n + 5/2

-65 â€“ 5/2 = – 5/2n

(-130 – 5)/2 = -5/2n

-135/2 = -5/2n

n = (-135/2) Ã— (-2/5)

n = -135/-5

n = 27

Therefore, -47 is 27^{th} term.

**6. (i) Check whether â€“ 150 is a term of the A.P. 11, 8, 5, 2, â€¦**

**Solution:-**

From the question it is given that,

The first term a = 11

Then, difference d = 8 â€“ 11 = -3

5 â€“ 8 = -3

2 â€“ 5 = -3

Then, common difference d = – 3

Let us assume -150 as n^{th} term,

T_{n} = a + (n – 1)d

So, -150 = 11 + (n – 1) (-3)

-150 = 11 â€“ 3n + 3

-150 = 14 – 3n

3n = 150 + 14

3n = 164

n = 164/3

n =

Therefore, â€“ 150 is not a term of the A.P. 11, 8, 5, 2, â€¦

(ii) Find whether 55 is a term of the A.P. 7, 10, 13, â€¦ or not. If yes, find which term is it.

**Solution:-**

From the question it is given that,

The first term a = 7

Then, difference d = 10 â€“ 7 = 3

13 â€“ 10 = 3

Then, common difference d = 3

Let us assume 55 as n^{th} term,

T_{n} = a + (n – 1)d

So, 55 = 7 + (n – 1)3

55 = 7 + 3n – 3

55 = 4 + 3n

3n = 55 – 4

3n = 51

n = 51/3

n = 17

Therefore, 55 is 17^{th} term of the A.P. 7, 10, 13, â€¦

**7. (i) Find the 20th term from the last term of the A.P. 3, 8, 13, â€¦, 253.**

**Solution:-**

Let us assume 253 as n^{th} term.

From the question,

The first term a = 3

Then, difference d = 8 â€“ 3 = 5

13 â€“ 8 = 5

Therefore, common difference d = 5

T_{n} = a + (n – 1)d

So, 253 = a + (n – 1)d

253 = 3 + (n – 1)5

253 = 3 + 5n – 5

253 = -2 + 5n

5n = 253 + 2

5n = 255

n = 255/5

n = 51

Therefore, 253 is 51^{th} term.

Now, assume â€˜Pâ€™ be the 20^{th} term from the last.

Then, P = L â€“ (n – 1)d

= 253 â€“ (20 – 1) 5

= 253 â€“ (19) 5

= 253 â€“ 95

P = 158

Therefore, 158 is the 20^{th} term from the last.

(ii) Find the 12^{th} from the end of the A.P. â€“ 2, â€“ 4, â€“ 6, â€¦, â€“ 100.

**Solution:-**

Let us assume -100 as n^{th} term.

From the question,

The first term a = -2

Then, difference d = – 4 â€“ (-2) = – 4 + 2 = -2

-6 â€“ (-4) = -6 + 4 = – 2

Therefore, common difference d = -2

T_{n} = a + (n – 1)d

So, – 100 = a + (n – 1)d

– 100 = -2 + (n – 1)(-2)

– 100 = -2 – 2n + 2

– 100 = -2n

n = -100/-2

n = 50

Therefore, -100 is 50^{th} term.

Now, assume â€˜Pâ€™ be the 12^{th} term from the last.

Then, P = L â€“ (n – 1)d

= -100 â€“ (12 – 1) (-2)

= -100 â€“ (11) (-2)

= -100 + 22

P = – 78

Therefore, -78 is the 12^{th} term from the last of the A.P. â€“ 2, â€“ 4, â€“ 6, â€¦

**8. Find the sum of the two middle most terms of the A.P.**

**-4/3, -1, -2/3, â€¦, **

** **

**Solution:-**

From the question,

Last term (n^{th}) =

** **= 13/3

First term a = -4/3

Then, difference d = -1 â€“ (-4/3) = -1 + 4/3 = (-3 + 4)/3 = 1/3

= -2/3 â€“ (-1) = -2/3 + 1 = (-2 + 3)/3 = 1/3

Therefore, common difference d = 1/3

We know that,

T_{n} = a + (n – 1)d

So, 13/3 = -4/3 + (n – 1)(1/3)

13/3 + 4/3 = 1/3n â€“ 1/3

13/3 + 4/3 + 1/3 = 1/3n

(13 + 4 + 1)/3 = 1/3n

18/3 = 1/3n

6 = 1/3n

n = 6 Ã— 3

n = 18

So, middle term is 18/2 and (18/2) + 1 = 9^{th} and 10^{th} term

Then, a_{9} + a_{10} = a + 8d + a + 9d

= 2a + 17d

Now substitute the value of â€˜aâ€™ and â€˜dâ€™.

= 2(-4/3) + 17(1/3)

= -8/3 + 17/3

= (-8 + 17)/3

= 9/3

= 3

Therefore, the sum of the two middle most terms of the A.P is 3.

**9. Which term of the A.P. 53, 48, 43,â€¦ is the first negative term ?**

**Solution:-**

From the question,

The first term a = 53

Then, difference d = 48 â€“ 53 = -5

= 43 â€“ 48 = -5

Therefore, common difference d = -5

T_{n} = a + (n – 1)d

= 53 + (n – 1)(-5)

= 53 – 5n + 5

= 58 – 5n

5n = 58

n= 58/5

n = 11.6 â‰ˆ 12

Therefore, 12^{th} term is the first negative term** **of the A.P. 53, 48, 43,â€¦

**10. Determine the A.P. whose third term is 16 and the 7 ^{th} term exceeds the 5^{th} term by 12.**

**Solution:-**

From the question it is given that,

T_{3} = 16

The 7th term exceeds the 5th term by 12 = T_{7} â€“ T_{5} = 12

We know that, T_{n} = a + (n – 1)d

So, T_{3} = a + 2d = 16 â€¦ [equation (i)]

T_{7} â€“ T_{5 }= (a + 6d) â€“ (a + 4d) = 12 â€¦ [equation (ii)]

12 = a + 6d â€“ a â€“ 4d

12 = 2d

d = 12/2

d = 6

Now, substitute value of d in equation (i) we get,

Then, T_{3} = a + 2d

16 = a + 2(6)

a = 16 â€“ 12

a = 4

Therefore, A.P. is 4 + 6 = 10, 10 + 6 = 16, 16 + 6 = 22

Hence, the four term of A.P. is 4, 10, 16, 22, â€¦.

**11. Find the 20 ^{th} term of the A.P. whose 7^{th} term is 24 less than the 11^{th} term, first term being 12.**

**Solution:-**

From the question it is given that,

First term a = 12

7th term is 24 less than the 11th term = T_{11} â€“ T_{7} = 24

T_{11} â€“ T_{7 }= (a + 10d) â€“ (a + 6d) = 24

24 = a + 10d â€“ a â€“ 6d

24 = 4d

d = 24/4

d = 6

Now, T_{20} = a + 19d

Substitute the values of a and d,

T_{20} = 12 + 19(6)

T_{20} = 12 + 114

T_{20} = 126

**12. Find the 31 ^{st} term of an A.P. whose 11^{th} term is 38 and 6^{th} term is 73.**

**Solution:-**

From the question it is given that,

T_{11} = 38

T_{6} = 73

Let us assume â€˜aâ€™ be the first term and â€˜dâ€™ be the common difference,

So, T_{11} = a + 10d = 38 equation (i)

T_{6} = a + 5d = 73 equation (ii)

Subtracting both equation (i) and equation (i),

(a + 10d) â€“ (a + 5d) = 73 â€“ 38

a + 10d â€“ a â€“ 5d = 35

5d = 35

d = 35/5

d = 7

now, substitute the value of d in equation (i) to find out a, we get

a + 10d = 38

a + 10(7) = 38

a + 70 = 38

a = 38 â€“ 70

a = -32

Therefore, T_{31} = a + 30d

= -32 + 30(7)

= -32 + 210

= 178

**13. If the seventh term of an A.P. is 1/9 and its ninth term is 1/7, find its 63 ^{rd} term.**

**Solution:-**

From the question it is given that,

T_{9} = 1/7

T_{7} = 1/9

Let us assume â€˜aâ€™ be the first term and â€˜dâ€™ be the common difference,

So, T_{9} = a + 8d = 1/7 equation (i)

T_{7} = a + 6d = 1/9 equation (ii)

Subtracting both equation (i) and equation (i),

(a + 6d) â€“ (a + 8d) = 1/9 â€“ 1/7

a + 6d â€“ a â€“ 8d = (7 – 9)/63

-2d = -2/63

d = (-2/63) Ã— (-1/2)

d = 1/63

now, substitute the value of d in equation (ii) to find out a, we get

a + 6(1/63) = 1/9

a = 1/9 â€“ 6/63

a = (7 – 6)/63

a = 1/63

Therefore, T_{63} = a + 62d

= 1/63 + 62(1/63)

= 1/63 + 62/63

= (1 + 62)/63

= 63/63

= 1

**14. (i) The 15 ^{th} term of an A.P. is 3 more than twice its 7^{th} term. If the 10^{th} term of the A.P. is 41, find its n^{th} term.**

**Solution:-**

From the question it I s given that,

T_{10} = 41

T_{10} = a + 9d = 41 â€¦ [equation (i)]

T_{15} = a + 14d = 2T_{7} + 3

= a + 14d = 2(a + 6d) + 3

= a + 14d = 2a + 12 d + 3

-3 = 2a â€“ a + 12d â€“ 14d

a â€“ 2d = -3 â€¦ [equation (ii)]

Now, subtracting equation (ii) from (i), we get,

(a + 9d) â€“ (a â€“ 2d) = 41 â€“ (-3)

a + 9d â€“ a + 2d = 41 + 3

11d = 44

d = 44/11

d = 4

Then, substitute the value of d is equation (i) to find a,

a + 9(4) = 41

a + 36 = 41

a = 41 â€“ 36

a = 5

Therefore, n^{th} term = T_{n} = a + (n – 1)d

= 5 + (n – 1)4

= 5 + 4n â€“ 4

= 1 + 4n

(ii) The sum of 5^{th} and 7^{th} terms of an A.P. is 52 and the 10^{th} term is 46. Find the A.P.

**Solution:-**

From the question it is given that,

a_{5} + a_{7} = 52

(a + 4d) + (a + 6d) = 52

a + 4d + a + 6d = 52

2a + 10d = 52

Divide both the side by 2 we get,

a + 5d = 26 â€¦ equation (i)

Given, a_{10} = a + 9d = 46

a + 9d = 46 â€¦ equation (ii)

Now subtracting equation (i) from equation (ii),

(a + 9d) â€“ (a + 5d) = 46 â€“ 26

a + 9d â€“ a â€“ 5d = 20

4d = 20

d = 20/4

d = 5

Substitute the value of d in equation (i) to find out a,

a + 5d = 26

a + 5(5) = 26

a + 25 = 26

a = 26 â€“ 25

a = 1

Then, a_{2} = a + d

= 1 + 5 = 6

a_{3} = a_{2} + d

= 6 + 5

= 11

a_{4} = a_{3} + d

= 11 + 5

= 16

Therefore, 1, 6, 11, 16,â€¦ are A.P.

**15. If 8 ^{th} term of an A.P. is zero, prove that its 38^{th} term is triple of its 18^{th} term.**

**Solution:-**

Froom the question it is given that,

T_{8} = 0

We have to prove that, 38^{th} term is triple of its 18^{th} term = T_{38} = 3T_{18}

T_{8} = a + 7d = 0

T_{8} = a = -7d

T_{38} = a + 37d

= -7d + 37d

= 30d

Take, T_{18} = a + 17d

Substitute the value of a and d,

T_{18} = -7d + 17d

T_{18} = 10d

By comparing results of T_{38} and T_{18}, 38^{th} term is triple of its 18^{th} term.

**16. Which term of the A.P. 3, 10, 17,â€¦ will be 84 more than its 13 ^{th} term?**

**Solution:-**

From the question it is given that,

First term a = 3

Common difference d = 10 â€“ 3 = 7

Then, T_{13} = a + 12d

= 3 + 12(7)

= 3 + 84

= 87

Let us assume that, n^{th} term is 84 more than its 13^{th} term

So, T_{n} = 84 + 87

= 171

We know that, T_{n} = a + (n – 1)d = 171

3 + (n – 1)7 = 171

3 + 7n â€“ 7 = 171

7n â€“ 4 = 171

7n = 171 + 4

7n = 175

n = 175/7

n = 25

**17. (i) How many two digit numbers are divisible by 3 ?**

**Solution:-**

The two digits numbers divisible by 3 are, 12, 15, 18, 21, 24,â€¦..,99.

The above numbers are A.P.

So, first number a = 12

Common difference d = 15 â€“ 12 = 3

Then, last number is 99

We know that, T_{n }(last number) = a + (n – 1)d

99 = 12 + (n – 1)3

99 = 12 + 3n â€“ 3

99 = 9 + 3n

99 â€“ 9 = 3n

3n = 90

n = 90/3

n = 30

Therefore, 30 two digits number are divisible by 3.

(ii) Find the number of natural numbers between 101 and 999 which are divisible by both 2 and 5.

**Solution:-**

The natural numbers which are divisible by both 2 and 5 are 110, 120, 130, 140, â€¦.,999

The above numbers are A.P.

So, first number a = 110

Common difference d = 120 â€“ 110 = 10

Then, last number is 999

We know that, T_{n }(last number) = a + (n – 1)d

999 = 110 + (n – 1)10

999 = 110 + 10n â€“ 10

999 = 100 + 10n

999 â€“ 100 = 10n

10n = 888

n = 888/10

n = 88

The number of natural numbers which are divisible by both 2 and 5 are 88.

**18. If the numbers n â€“ 2, 4n â€“ 1 and 5n + 2 are in A.P., find the value of n.**

**Solution:-**

From the question it is given that, n â€“ 2, 4n â€“ 1 and 5n + 2 are in A.P.

Multiplying by 2 to 4n â€“ 1 then it becomes = 8n â€“ 2

So, 8n â€“ 2 = n â€“ 2 + 5n + 2

8n â€“ 2 = 6n

8n â€“ 6n = 2

2n = 2

n = 2/2

n = 1

**19. The sum of three numbers in A.P. is 3 and their product is â€“ 35. Find the numbers.**

**Solution:-**

From the question it is given that,

The sum of three numbers in A.P. = 3

Given, Their product = -35

Let us assume the 3 numbers which are in A.P. are, a â€“ d, a, a + d

Now adding 3 numbers = a â€“ d + a + a + d = 3

3a = 3

a = 3/3

a = 1

From the question, product of 3 numbers is â€“ 35

So, (a – d) Ã— (a) Ã— (a + d) = – 35

(1 – d) Ã— (1) Ã— (1 + d) = – 35

1^{2} â€“ d^{2} = – 35

d^{2} = 35 + 1

d^{2} = 36

d = âˆš36

d = Â±6

Therefore, the numbers are (a – d) = 1 â€“ 6 = – 5

a = 1

(a + d) = 1 + 6 = 7

If d = – 6

The numbers are (a – d) = 1 â€“ (-6) = 1 + 6 = 7

a = 1

(a + d) = 1 + (-6) = 1 â€“ 6 = -5

Therefore, the numbers -5, 1, 7,â€¦ and 7, 1, -5,â€¦ are in A.P.

**20. The sum of three numbers in A.P. is 30 and the ratio of first number to the third number is 3 : 7. Find the numbers.**

**Solution:-**

From the question it is given that, sum of three numbers in A.P. = 30

The ratio of first number to the third number is 3: 7

Let us assume the 3 numbers which are in A.P. are, a â€“ d, a, a + d

Now adding 3 numbers = a â€“ d + a + a + d = 30

3a = 30

a = 30/3

a = 10

Given ratio 3 : 7 = a â€“ d : a + d

3/7 = (a – d)/(a + d)

(a – d)7 = 3(a + d)

7a â€“ 7d = 3a + 3d

7a â€“ 3a = 7d + 3d

4a = 10d

4(10) = 10d

40 = 10d

d = 40/10

d = 4

Therefore, the numbers are a â€“ d = 10 â€“ 4 = 6

a = 10

a + d = 10 + 4 = 14

So, 6, 10, 14, â€¦ are in A.P.

**21. The sum of the first three terms of an A.P.is 33. If the product of the first and the third terms exceeds the second term by 29, find the A.P.**

**Solution:-**

From the question it is given that, sum of the first three terms of an A.P. is 33.

Let us assume the 3 numbers which are in A.P. are, a â€“ d, a, a + d

Now adding 3 numbers = a â€“ d + a + a + d = 33

3a = 33

a = 33/3

a = 11

Given, the product of the first and the third terms exceeds the second term by 29.

(a – d) (a + d) = a + 29

a^{2} â€“ d^{2} = 11 + 29

11^{2} â€“ d^{2} = 40

121 â€“ 40 = d^{2}

d^{2} = 81

d =âˆš81

d = Â±9

If d = 9

Therefore, the numbers are (a – d) = 11 â€“ 9 = 2

a = 11

(a + d) = 11 + 9 = 20

If d = – 9

The numbers are (a – d) = 1 â€“ (-9) = 11 + 9 = 20

a = 11

(a + d) = 11 + (-9) = 11 â€“ 9 = 2

Therefore, the numbers 2, 11, 20,â€¦ and 20, 11, 2,â€¦ are in A.P.

Exercise 9.3

**1. Find the sum of the following A.P.s :(i) 2, 7, 12, â€¦ to 10 terms**

**Solution:-**

From the question,

First term a = 2

Then, d = 7 â€“ 2 = 5

12 â€“ 7 = 5

So, common difference d = 5

n = 10

S_{10} = n/2(2a + (n – 1)d)

= 10/2 ((2 Ã— 2) + (10 – 1)5)

= 5(4 + 45)

= 5(49)

= 245

**(ii) 1/15, 1/12, 1/10, â€¦ to 11 terms**

**Solution:-**

From the question,

First term a = 1/15

Then, d = 1/12 â€“ 1/15

= (5 – 4)/60

= 1/60

So, common difference d = 1/60

n = 11

S_{11} = 11/2(2a + (n – 1)d)

= 11/2 ((2 Ã— (1/15)) + (11 – 1)(1/60))

= 11/2 ((2/15) + (10/60))

= 11/2 (2/15 + 1/6)

= 11/2 (4 + 5)/30

= 11/2 (9/30)

= 11/2(3/10)

= 33/20

**2. Find the sums given below :(i) 34 + 32 + 30 + â€¦ + 10**

**Solution:-**

From the question,

First term a = 34,

Difference d = 32 â€“ 34 = -2

So, common difference d = – 2

Last term T_{n} = 10

We know that, T_{n} = a + (n – 1)d

** **10 = 34 + (n – 1)(-2)

-24 = – 2(n – 1)

-24 = – 2n + 2

2n = 24 + 2

2n = 26

n = 26/2

n = 13

S_{n} = n/2(a + 1)

= 13/2 (34 + 10)

= 13/2 (44)

= 13 (22)

= 286

(ii) â€“ 5 + ( â€“ 8) + ( â€“ 11) + â€¦ + ( â€“ 230)

**Solution:-**

From the question,

First term a = -5,

Difference d = -8 â€“ (-5) = -8 + 5 = -3

So, common difference d = – 3

Last term T_{n} = -230

We know that, T_{n} = a + (n – 1)d

** **-230 = -5 + (n – 1)(-3)

-230 = – 5 â€“ 3n + 3

-230 = – 2 â€“ 3n

3n = 230 -2

3n = 228

n = 228/3

n = 76

Therefore, S_{n} = n/2 (a + l)

= 76/2 (-5 + (-230))

= 38 (-5 – 230)

= 38 (235)

= – 8930

**3. In an A.P. (with usual notations) :(i) given a = 5, d = 3, a _{n}Â = 50, find n and S_{n}**

**Solution:-**

From the question,

First term a = 5

Then common difference d = 3

a_{n} = 50,

We know that, a_{n} = a + (n – 1)d

50 = 5 + (n – 1)3

50 = 5 + 3n â€“ 3

50 = 2 + 3n

3n = 50 â€“ 2

3n = 48

n = 48/3

n = 16

So, S_{n} = (n/2)(2a + (n – 1)d)

= (16/2) ((2 Ã— 5) + (16 – 1) Ã— 3)

= 8(10 + 45)

= 8(55)

= 440

(ii) given a = 7, a_{13}Â = 35, find d and S_{13}

**Solution:-**

From the question,

First term a = 7

a_{13} = 35,

We know that, a_{n} = a + (n – 1)d

35 = 7 + (13 – 1)d

35 = 7 + 13d â€“ d

35 = 7 + 12d

12d = 35 â€“ 7

12d = 28

d = 28/12 â€¦ [divide by 4]

d = 7/3

So, S_{13} = (n/2)(2a + (n – 1)d)

= (13/2) ((2 Ã— 7) + ((13 – 1) Ã— (7/3))

= (13/2) ((14 + (12 Ã— 7/3))

= (13/2) (14 + 28)

= (13/2) (42)

= 13 Ã— 21

= 273

(iii) given d = 5, S_{9}Â = 75, find a and a_{9}.

**Solution:-**

From the question it is given that,

Common difference d = 5

S_{9} = 75

We know that, a_{n} = a + (n – 1)d

a_{9} = a + (9 – 1)5

a_{9} = a + 45 â€“ 5

a_{9} = a + 40 â€¦ [equation (i)]

Then, S_{9} = (n/2) (2a + (n – 1)d)

75 = (9/2) (2a + (9 – 1)5)

75 = (9/2) (2a + (8)5)

(75 Ã— 2)/9 = 2a + 40

150/9 = 2a + 40

2a = 150/9 â€“ 40

2a = 50/3 â€“ 40

2a = (50 – 120)/3

2a = -70/3

a = -70/(3 Ã— 2)

a = – 35/3

Now, substitute the value of a in equation (i),

a_{9} =a + 40

= -35/3 + 40

= (-35 + 120)/3

= 85/3

(iv) given a = 8, a_{n}Â = 62, S_{n}Â = 210, find n and d

**Solution:-**

From the question it is give that,

First term a = 8,

a_{n }= 62 and S_{n} = 210

We know that, a_{n} = a + (n – 1)d

62 = 8 + (n – 1)d

(n – 1)d = 62 â€“ 8

(n – 1)d = 54 â€¦ [equation (i)]

Then, S_{n} = (n/2) (2a + (n – 1)d)

210 = (n/2) ((2 Ã— 8) + 54) â€¦ [from equation (i) (n – 1)d = 54]

210 = (n/2) (16 + 54)

420 = n(70)

n = 420/70

n = 6

Now, substitute the value of n in equation (i),

(n – 1)d = 54

(6 – 1)d = 54

5d = 54

d = 54/5

Therefore, d = 54/5 and n = 6

(v) given a = 3, n = 8, S = 192, find d.

**Solution:-**

From the question it is given that,

First term a = 3

n = 8

S = 192

We know that, S_{n} = (n/2) (2a + (n – 1)d)

192 = (8/2) ((2 Ã— 3) + (8 – 1)d)

192 = 4 (6 + 7d)

192/4 = 6 + 7d

48 = 6 + 7d

48 â€“ 6 = 7d

42 = 7d

d = 42/7

d = 6

Therefore, common difference d is 6.

**4. (i) The first term of an A.P. is 5, the last term is 45 and the sum is 400. Find the number of terms and the common difference.**

**Solution:-**

From the question it is give that,

First term a = 5

Last term = 45

Then, sum = 400

We know that, last term = a + (n â€“ 1)d

45 = 5 + (n – 1)d

(n – 1)d = 45 â€“ 5

(n – 1)d = 40 â€¦ [equation (i)]

So, S_{n} = (n/2) (2a + (n – 1)d)

400 = (n/2) ((2 Ã— 5) + 40) â€¦ [from equation (i) (n – 1)d = 40]

800 = n(10 + 40)

800 = 50n

n = 800/50

n = 16

(ii) The sum of first 15 terms of an A.P. is 750 and its first term is 15. Find its 20th term.

**Solution:-**

From the question it is give that,

First term a = 15

Therefore, sum of first n terms of an A.P. is given by,

S_{n} = (n/2) (2a + (n – 1)d)

S_{15} = (15/2)(2a + (15 – 1)d)

750 = (15/2) (2a + 14d)

(750 Ã— 2)/15 = 2a + 14d

100 = 2a + 14d

Dividing both the side by 2 we get,

50 = a + 7d

Now, substitute the value a,

50 = 15 + 7d

7d = 50 â€“ 15

7d = 35

d = 35/7

d = 5

So, 20^{th} term a_{20} = a + 19d

= 15 + 19(5)

= 15 + 95

= 110

**5. The first and the last terms of an A.P. are 17 and 350 respectively. If the common difference is 9, how many terms are there and what is their sum?**

**Solution:-**

From the question it is give that,

First term a = 17

Last term (l) = 350

Common difference d = 9

We know that, l = T_{n} = a + (n – 1)d

350 = 17 + (n – 1) Ã— 9

350 â€“ 17 = 9n â€“ 9

333 + 9 = 9n

342 = 9n

n = 342/9

n = 38

So, S_{n} = (n/2) (2a + (n – 1)d)

= (38/2) ((2 Ã— 17) + (38 – 1)d)

= 19(34 + (37 Ã— 9))

= 19(34 + 333)

= 19 Ã— 367

= 6973

Therefore, n = 38 and S_{n} = 6973

**6. Solve for x : 1 + 4 + 7 + 10 + â€¦ + x = 287.**

**Solution:-**

From the question,

First term a = 1

Difference d = 4 â€“ 1 = 3

n = x

x = a = (n – 1)d

x â€“ 1 = (n – 1)d

S_{n} = (n/2) (2a + (n – 1)d)

287 = (n/2) ((2 Ã— 1)+ (n – 1)3)

= n (2 + 3n – 3)

574 = n(2 + 3n – 3)

574 = 2n + 3n^{2} â€“ 3n

574 = – n + 3n^{2}

3n^{2} â€“ n â€“ 574 = 0

3n^{2} â€“ 42n + 41 â€“ 574 = 0

3n(n – 14) + 41(n – 14) = 0

(n – 14) (3n + 41) = 0

If n â€“ 14 = 0

n = 14

or 3n + 41 = 0

3n = -41

n = -41/3

We have to take positive number so n = 14

Then, = a + (n – 1)d

= 1 + (14 – 1) 3

= 1 + (13)3

= 1 + 39

= 40

Therefore, x = 40

7. (i) How many terms of the A.P. 25, 22, 19, â€¦ are needed to give the sum 116 ? Also find the last term.

**Solution:-**

From the question it is given that,

First term a = 25

Common difference d = 22 â€“ 25 = – 3

Sum = 116

S_{n} = (n/2) (2a + (n – 1)d)

116 = (n/2) (2a + (n – 1)d)

By cross multiplication,

232 = n ((2 Ã— 25) + (n – 1) (-3))

232 = n (50 â€“ 3n + 3)

232 = n (53 â€“ 3n)

232 = 53n â€“ 3n^{2}

3n^{2} â€“ 53n + 232 = 0

3n^{2} â€“ 24n â€“ 29n + 232 = 0

3n (n – 8) â€“ 29 (n – 8) = 0

(n – 8) (3n – 29) = 0

If n â€“ 8 = 0

n = 8

or 3n – 29 = 0

3n = 29

n = 29/3

not possible to take fraction,

So, n = 8

Then, T = a + (n – 1)d

= 25 +(8 – 1) (-3)

= 25 + 7 (-3)

= 25 â€“ 21

= 4

(ii) How many terms of the A.P. 24, 21, 18, â€¦ must be taken so that the sum is 78 ? Explain the double answer.

**Solution:-**

From the question it is given that,

First term a = 24

Common difference d = 21 â€“ 24 = – 3

Sum = 78

S_{n} = (n/2) (2a + (n – 1)d)

78 = (n/2) (2a + (n – 1)d)

By cross multiplication,

156 = n ((2 Ã— 24) + (n – 1) (-3))

156 = n (48 â€“ 3n + 3)

156 = n (51 â€“ 3n)

156 = 51n â€“ 3n^{2}

3n^{2} â€“ 51n + 156 = 0

3n^{2} â€“ 12n â€“ 39n + 156 = 0

3n (n – 4) â€“ 39 (n – 4) = 0

(n – 4) (3n – 39) = 0

If n â€“ 4 = 0

n = 4

or 3n – 39 = 0

3n = 39

n = 39/3

n = 13

now we have to consider both values

So, n = 4

Then, T = a + (n – 1)d

= 24 +(4 – 1) (-3)

= 24 + 3 (-3)

= 24 â€“ 9

= 15

n = 13

Then, T = a + (n – 1)d

= 24 +(13 – 1) (-3)

= 24 + 12 (-3)

= 24 â€“ 36

= -12

So, (12 + 9 + 6 + 3 + 0 + (-3)+ (-6) + (-9) + (-12)) = 0

Hence, the sum of 5^{th} term to 13^{th} term = 0

**8. Find the sum of first 22 terms, of an A.P. in which d = 7 and a _{22}Â is 149.**

**Solution:-**

From the question it is given that,

Common difference d = 7

a_{22} = 149

n = 22

we know that,

a_{22} = (n – 1)d

149 = a + (22 – 1)7

149 = a + (22)7

149 = a + 147

a = 149 â€“ 147

a = 2

So, S_{22 }= (n/2) (2a + (n – 1)d)

= (22/2) ((2 Ã— 2) + (22 – 1)7)

= 11(4 + (21)7)

= 11 (4 + 147)

= 11 (151)

= 1661

**9. In an A.P., the fourth and sixth terms are 8 and 14 respectively. Find the:**

**(i) first term (ii) common difference**

**(iii) sum of the first 20 terms.**

**Solution: **

From the question it is given that,

T_{4} = 8 and T_{6} = 14

â‡’ a + 3d = 8 â€¦ (i)

â‡’ a + 5d = 14 â€¦ (ii)

Subtracting (i) from (ii), we get

(a + 5d) – (a + 3d) = 14 – 8

5d â€“ 3d = 6

2d = 6

d = 6/2

d = 3

(ii) Hence, common difference d = 3

Substituting the value of d in (i), we have

a + 3(3) = 8

a = 8 â€“ 9

a = -1

(i) Hence, the first term = -1

(iii) The sum of first 20 terms

n = 20

S_{20} = n/2 Ã— [2a + (n – 1)d]

= 20/2 Ã— [2(-1) + (20 – 1)(3)]

= 10 Ã— [-2 + (19)(3)]

= 10 Ã— [-2 + 57]

= 10 Ã— 55

= 550

**10. (i) Find the sum of first 51 terms of the A.P. whose second and third terms are 14 and 18 respectively.**

**Solution:-**

From the question it is given that,

T_{2} = 14, T_{3} = 18

So, common difference d = T_{3} â€“ T_{2}

= 18 â€“ 14

= 4

Where, a = T_{1} = 14 â€“ 4 = 10

n = 51

We know that,

S_{51 }= (n/2) (2a + (n – 1)d)

= (51/2) ((2 Ã— 10) + (51 – 1)4)

= (51/2) (20 + (50 Ã— 4))

= (51/2) (20 + 200)

= (51/2) Ã— 220

= 5610

(ii) The 4^{th} term of A.P is 22 and 15^{th} term is 66. Find the first term and the common difference. Hence, find the sum of first 8 term of the A.P.

**Solution:**

From the question it is given that,

T_{4} = 22, T_{15} = 66

â‡’ a + 3d = 22 â€¦ (i)

â‡’ a + 14d = 66 â€¦ (ii)

Subtracting (i) from (ii), we get

(a + 14d) – (a + 3d) = 66 – 22

14d â€“ 3d = 44

11d = 44

d = 44/11

d = 4

So, common difference d = 4

Substituting the value of d in (i), we have

a + 3(4) = 22

a = 22 â€“ 12

a = 10

Hence, the first term a = 10

Now, the sum of first 8 terms of the A.P is

n = 8

S_{n} = n/2 Ã— [2a + (n – 1)d]

S_{8} = 8/2 Ã— [2(10) + (8 – 1)(4)]

= 10 Ã— [20 + (7)(4)]

= 10 Ã— [20 + 28]

= 10 Ã— 48

= 480

**11. If the sum of first 6 terms of an A.P. is 36 and that of the first 16 terms is 256, find the sum of first 10 terms.**

**Solution:-**

From the question it is given that,

S_{6} = 36

S_{16} = 256

We know that,

S_{n} = (n/2) (2a + (n – 1)d)

S_{6} = (6/2) (2a + (6 – 1)d) = 36

3 (2a + 5d) = 36

Divide both the side by 3,

2a + 5d = 12 â€¦ [equation (i)]

Now, S_{16} = (16/2) (2a + (16 – 1)d) = 256

8 (2a + 15d) = 256

Divide both the side by 8,

2a + 15d = 32 â€¦ [equation (ii)]

Then, subtract equation (ii) from equation (i) we get,

(2a + 5d) â€“ (2a + 15d) = 12 â€“ 32

2a + 5d â€“ 2a â€“ 15d = -20

-10d = -20

d = -20/-10

d = 2

substitute the value of d in equation (i) to find a,

2a + 5d = 12

2a + 5(2) = 12

2a + 10 = 12

2a = 12 â€“ 10

2a = 2

a = 2/2

a = 1

So, S_{10} = (n/2) (2a + (n – 1)d)

= (10/2) ((2 Ã— 1) + (10 – 1)2)

= 5 (2 + 18)

= 5 (20)

= 100

Therefore, the sum of first 10 terms is 100.

**12. Show that a _{1}, a_{2}, a_{3}, â€¦ form an A.P. where a_{n}Â is defined as a_{n}Â = 3 + 4n. Also find the sum of first 15 terms.**

**Solution:-**

From the question it is given that,

n^{th} term is 3 + 4n

So, a_{n} = 3 + 4n

Now, we start giving values, 1, 2, 3, â€¦ in the place of n, we get,

a_{1} = 3 + (4 Ã— 1) = 3 + 4 = 7

a_{2} = 3 + (4 Ã— 2) = 3 + 8 = 11

a_{3} = 3 + (4 Ã— 3) = 3 + 12 = 15

a_{4} = 3 + (4 Ã— 4) = 3 + 16 = 19

So, The numbers are 7, 11, 15, 19, â€¦.

Then, first term a = 7, common difference d = 11 â€“ 7 = 4

We know that,

S_{15} = (n/2) (2a + (n – 1)d)

= (15/2) ((2 Ã— 7) + (15 – 1) Ã— 4)

= (15/2) (14 + (14 Ã— 4))

= (15/2) (14 + 56)

= (15/2) Ã— 70

= 525

Therefore, the sum of first 15 terms is 525.

**13. The sum of first six terms of an arithmetic progression is 42. The ratio of the 10 ^{th} term to the 30^{th} term is 1: 3. Calculate the first and the thirteenth term. **

**Solution: **

Given,

S_{6} = 42 and T_{10}/T_{30} = 1/3

We know that,

S_{n} = (n/2) (2a + (n – 1)d)

T_{n} = a + (n – 1)d

So, we have

S_{6} = (6/2) Ã— [2a + (6 – 1)d] = 42

3 Ã— (2a + 5d) = 42

2a + 5d = 14 â€¦ (i) [Dividing by 3]

And,

T_{10}/T_{30} = [a + (10 – 1)d]/ [a + (30 – 1)d] = 1/3

(a + 9d)/ (a + 29d) = 1/3

On cross-multiplication, we have

3(a + 9d) = a + 29d

3a + 27d = a + 29d

3a â€“ a + 27d â€“ 29d = 0

2a â€“ 2d = 0

a â€“ d = 0

a = d

Using the above the relation in (i), we get

2a + 5a = 14

7a = 14

a = 14/7

a = 2

Hence, the first term of the A.P is 2.

Now, the thirteenth term is given by

T_{13} = 2 + (13 – 1)(2)

= 2 + 12 Ã— 2

= 2 + 24

= 26

**14. In an A.P., the sum of its first n terms is 6n â€“ n ^{2}. Find its 25^{th} term. **

**Solution: **

Given,

S_{n} = 6n â€“ n^{2}

Now,

S_{1} = 6(1) â€“ (1)^{2} = 6 â€“ 1 = 5

So, the first term a = 5

And,

S_{2} = 6(2) â€“ (2)^{2} = 12 â€“ 4 = 8

So, T_{1} + T_{2}^{ }= 8

5 + T_{2} = 8

T_{2} = 8 â€“ 5 = 3

So, the common difference d = T_{2} â€“ T_{1}

d = 3 â€“ 5

= -2

Thus, 25^{th} term in the A.P is given by

T_{25} = 5 + (25 – 1)(-2)

= 5 + 24(-2)

= 5 â€“ 48

= -43

**15. If S _{n} denotes the sum of first n terms of an A.P., prove that S_{30} = 3 (S_{20} â€“ S_{10}).**

**Solution: **

We know that,

S_{n} = n/2 Ã— [2a + (n – 1)d]

So,

S_{10} = 10/2 Ã— [2a + (10 – 1)d]

= 5 Ã— (2a + 9d)

= 10a + 45d

S_{20} = 20/2 Ã— [2a + (20 – 1)d]

= 10 Ã— (2a + 19d)

= 20a + 190d

S_{30} = 30/2 Ã— [2a + (30 – 1)d]

= 15 Ã— (2a + 29d)

= 30a + 435d

Now, taking L.H.S we have

3 (S_{20} â€“ S_{10}) = 3 [20a + 190d â€“ (10a + 45d)]

= 3 (20a + 190d â€“ 10a â€“ 45d)

= 3 (10a + 145d)

= 30a + 435d

= S_{30}

= R.H.S

– Hence proved

**16. (i) Find the sum of first 1000 positive integers. **

**(ii) Find the sum of first 15 multiples of 8. **

**Solution: **

(i) First 1000 positive integers are:

1, 2, 3, 4, â€¦â€¦.., 1000

This is an A.P with first term a = 1 and common difference d = 1

We know that,

S_{n} = n/2 Ã— [2a + (n – 1)d]

Thus,

S_{1000} = 1000/2 Ã— [2(1) + (1000 – 1)(1)]

= 500 Ã— (2 + 99)

= 500 Ã— 101

= 50500

Therefore, the** **sum of first 1000 positive integers is 50500.

(ii) The first 15 multiples of 8 are:

8, 16, 24, â€¦.. where n = 15

This forms an A.P with first term a = 8 and common difference d = 16 â€“ 8 = 8

We know that,

S_{n} = n/2 Ã— [2a + (n – 1)d]

Thus,

S_{15} = 15/2 Ã— [2(8) + (14 – 1)(8)]

= 15/2 Ã— (16 + 13 Ã— 8)

= 15/2 Ã— (16 + 104)

= 15/2 Ã— 120

= 15 Ã— 60

= 900

Therefore, the sum of first 15 multiples of 8 is 900.

**17. (i) Find the sum of all two digit natural numbers which are divisible by 4. **

**(ii) Find the sum of all natural numbers between 100 and 200 which are divisible by 4. **

**(iii) Find the sum of all multiples of 9 lying between 300 and 700. **

**(iv) Find the sum of all natural numbers less than 100 which are divisible by 4. **

**Solution: **

(i) The two-digit natural numbers which are divisible by 4 are:

4, 8, 12, 16, â€¦..

This form an A.P.

The last term in this series is found out by dividing 100 by 4

100/4 = 25 and remainder is zero

So, the last two-digit number which is divisible by 4 is 96

And, it is the 24^{th} term

So, we have

a = 4, d = 4, last term l = 96 and n = 24

Thus, sum of these numbers is

S_{24} = 24/2 (4 + 96)

= 12 Ã— 100

= 1200

(ii) The natural numbers between 100 and 200 which are divisible by 4 are:

104, 108, 112, â€¦..

This form an A.P.

The last term in this series is found out by dividing 200 by 4

200/4 = 50 and remainder is zero

So, the last natural number between 100 and 200 which is divisible by 4 is 196

So, we have

a = 104, d = 4 and last term l = 96

The number of terms is found out by

T_{n} = 196 = 104 + (n – 1)(4)

196 = 104 + 4n â€“ 4

4n = 196 â€“ 104 + 4

4n = 96

n = 96/4

n = 24

Thus, sum of these numbers is

S_{24} = 24/2 (104 + 196)

= 12 Ã— 200

= 2400

(iii) The multiples of 9 lying between 300 and 700 are:

306, 315, 324, â€¦â€¦

This form an A.P. where a = 306 and d = 9

The last term in this series is found out by dividing 700 by 9

700/9 gives 77 as quotient and 7 as remainder.

So, the last number between 300 and 700 which is a multiple is 700 â€“ 7 = 693

Now, we have

a = 306, d = 9 and l = 693

The number of terms is found out by

T_{n} = 693 = 306 + (n – 1)(9)

693 = 306 + 9n â€“ 9

9n = 693 â€“ 306 + 9

9n = 396

n = 396/9

n = 44

Thus, sum of these numbers is

S_{44} = 44/2 (306 + 693)

= 22 Ã— 999

= 21978

(iv) The natural numbers less than 100 which are divisible by 4 are:

4, 8, 12, â€¦., 96

This forms an A.P. where a = 4, d = 4 and l = 96

Now, for calculating n

T_{n} = 96 = 4 + (n – 1)4

96 = 4 + 4n â€“ 4

4n = 96

n = 96/4

n = 24

Thus, sum of these numbers is

S_{24} = 24/2 (4 + 96)

= 12 Ã— 100

= 1200

Exercise 9.4

**1. (i) Find the next term of the list of numbers 1/6, 1/3, 2/3, â€¦**

**Solution:-**

From the question,

First term a = 1/6

Then, r = (1/3) Ã· (1/6)

r = (1/3) Ã— (6/1)

r = 6/3

r = 2

Therefore, next term = 2/3 Ã— 2 = 4/3

**(ii) Find the next term of the list of numbers 3/16, -3/8, Â¾, -3/2,â€¦**

**Solution:-**

From the question,

First term a = 3/16

Then, r = (-3/8) Ã· (3/16)

r = (-3/8) Ã— (16/3)

r = (-3 Ã— 16)/(8 Ã— 3)

r = (-1 Ã— 2)/ (1 Ã— 1)

r = -2

Therefore, next term = -3/2 Ã— (-2) = 6/2 = 3

**(iii) Find the 15 ^{th} term of the series âˆš3 + 1/âˆš3 + 1/3âˆš3 + â€¦**

**Solution:-**

From the question,

First term a = âˆš3

Then, r = (1/âˆš3) Ã· (âˆš3)

r = (1/âˆš3) Ã— (1/âˆš3)

r = (1 Ã— 1)/( âˆš3 Ã— âˆš3)

r = 1/(âˆš3)^{2}

r = 1/3

So, a_{15} = ar^{n â€“ 1}

= âˆš3(1/3)^{15 â€“ 1}

= âˆš3(1/3)^{14}

= âˆš3 Ã— (1/3^{14})

Therefore, a_{15}^{ }= âˆš3 Ã— (1/3^{14})

**(iv) Find the n ^{th} term of the list of numbers 1/âˆš2, -2, 4âˆš2, – 16,â€¦**

**Solution:-**

From the question it is given that,

First term a = 1/âˆš2

Then, r = -2 Ã· (1/âˆš2)

r = (-2/1) Ã— (âˆš2/1)

r = (-2 Ã— âˆš2)/(1 Ã— 1)

r = -2âˆš2

So, a_{n} = ar^{n â€“ 1}

= (1/âˆš2)(-2âˆš2)^{n â€“ 1}

= (1/âˆš2) Ã— (-1)^{n – 1} Ã— [(âˆš2)^{2} Ã— âˆš2]^{n – 1}

= (-1)^{n â€“ 1} Ã— 1/âˆš2 Ã— [(âˆš2)^{3}]^{n }^{â€“ 1}

** **= (-1)^{n â€“ 1} Ã— 1/âˆš2 Ã— (âˆš2)^{3n }^{â€“ 3}

** **= (-1)^{n â€“ 1} (âˆš2)^{3n }^{â€“ 3 â€“ 1}

** **= (-1)^{n â€“ 1} (âˆš2)^{3n }^{â€“ 4}

** **= (-1)^{n â€“ 1} Ã— 2^{(3n }^{â€“ }^{4)/2}

Therefore, a_{n}^{ }= (-1)^{n â€“ 1} Ã— 2^{(3n }^{â€“ }^{4)/2}

**(v) Find the 10 ^{th} and n^{th} terms of the list of numbers 5, 25, 125, â€¦**

**Solution:-**

From the question it is given that,

First term a = 5,

Then, r = (25) Ã· (5)

r = (25) Ã— (1/5)

r = 5

So, a_{10} = ar^{n â€“ 1}

= 5 Ã— (5)^{10 â€“ 1}

= 5 Ã— 5 ^{9}

= 5^{9 + 1} â€¦ [by a^{m} Ã— a^{n} = a^{m + n}]

= 5^{10}

Therefore, a_{n} = ar^{n â€“ 1}

= 5 Ã— 5^{n â€“ 1}

= 5^{n â€“ 1 + 1}

= 5^{n}

**(vi) Find the 6 ^{th} and the n^{th} terms of the list of numbers 3/2, Â¾, 3/8,â€¦**

**Solution:-**

From the question it is given that,

First term a = 3/2,

Then, r = (3/4) Ã· (3/2)

r = (3/4) Ã— (2/3)

r = (3 Ã— 2)/(4 Ã— 3)

r = (1 Ã— 1)/(2 Ã— 1)

r = Â½

So, a_{n} = ar^{n â€“ 1}

= (3/2) Ã— (1/2)^{n â€“ 1}

= 3 Ã— Â½ Ã— (Â½)^{n – 1}

= 3 Ã— (Â½)^{n â€“ 1 + 1}

= 3 Ã— (Â½)^{n}

= 3/2^{n}

Therefore, a_{6} = 3/2^{n}

= 3/2^{6}

= 3/64

**(vii) Find the 6th term from the end of the list of numbers 3, â€“ 6, 12, â€“ 24, â€¦, 12288.**

**Solution:-**

From the question it is given that,

Last term = 12288

First term a = 3,

Then, r = (-6) Ã· (3)

r = (-6) Ã— (1/3)

r = (-6 Ã— 1)/(1 Ã— 3)

r = (-2 Ã— 1)/(1 Ã— 1)

r = -2

Then, 6^{th} term from the end,

a_{6} = l Ã— (1/r)^{n â€“ 1}

= 12288 Ã— (1/-2)^{6 â€“ 1}

= 12288 Ã— (1/-2^{5})

= 12288/-32

= – 384

**2. Which term of the G.P.**

**(i) 2, 2âˆš2, 4, â€¦ is 128?**

**Solution:-**

From the question it is given that,

Last term = 128

First term a = 2,

Then, r = (2âˆš2) Ã· (2)

r = (2âˆš2)/2

r = âˆš2

Then, a_{n} = ar^{n â€“ 1}

So, 128 = 2(âˆš2)^{n â€“ 1}

2^{7} = 2(âˆš2)^{n â€“ 1}

2^{7}/2 = (âˆš2)^{n â€“ 1}

2^{7 â€“ 1} = (âˆš2)^{n â€“ 1}

2^{6} = (âˆš2)^{n â€“ 1}

(âˆš2)^{n -1} = (âˆš2)^{12}

Now, comparing the powers

n â€“ 1 = 12

n = 12 + 1

n = 13

Therefore, 128 is the 13^{th} term.

**(ii) 1, 1/3, 1/9, â€¦ is 1/243**

**Solution:-**

From the question it is given that,

Last term (a_{n}) = 1/243

First term a = 1,

Then, r = (1/3) Ã· (1)

r = (1/3) Ã— (1/1)

r = 1/3

Then, a_{n} = ar^{n â€“ 1}

** **1/243 = 1 Ã— (1/3)^{n â€“ 1}

(1/3)^{5} = (1/3)^{n â€“ 1}

By comparing both left hand side and right hand side,

5 = n â€“ 1

n = 5 + 1

n = 6

Therefore, 1/243 is 6^{th} term.

**3. Determine the 12 ^{th} term of a G.P. whose 8^{th} term is 192 and common ratio is 2.**

**Solution:-**

From the question it is given that,

a_{8} = 192 and r = 2

Then, by the formula a_{n} = ar^{n â€“ 1 }

a_{8} = ar^{8 â€“ 1}

192 = a(2)^{8 â€“ 1}

192 = a(2)^{7}

a = 192/2^{7}

a = 192/128

a = 3/2

Now, a_{12} = (3/2)(2)^{12 â€“ 1}

= (3/2) Ã— (2)^{11}

= (3/2) Ã— 2048

= 3072

a_{8} = 3072

**4. In a GP., the third term is 24 and 6 ^{th} term is 192. Find the 10^{th} term.**

**Solution:-**

From the question it is given that,

a_{3} = 24

a_{6} = 192

Then, by the formula a_{n} = ar^{n â€“ 1 }

a_{6} = ar^{6 â€“ 1}

192 = ar^{6 â€“ 1}

192 = ar^{5} â€¦ [equation (i)]

Now, a_{3} = ar^{n â€“ 1}

24 = ar^{3 â€“ 1}

24 = ar^{2} â€¦ [equation (ii)]

By dividing equation (i) by equation (ii)

ar^{5}/ar^{2} = 192/24

r^{5 â€“ 2} = 8

r^{3} = 8

r^{3} = 2^{3}

r = 2

Now, substitute the value r in equation (i),

192 = ar^{5}

192 = a (2)^{5}

a = 192/32

a = 6

So, a_{10} = ar^{10 â€“ 1}

= ar^{9}

= 6(2)^{9}

= 6 (512)

= 3072

**5. Find the number of terms of a G.P. whose first term is Â¾, common ratio is 2 and the last term is 384.**

**Solution:-**

From the question it is given that,

First term of G.P. a = Â¾

Common ratio (r) = 2

Last term = 384

Then, by the formula a_{n} = ar^{n â€“ 1 }

384 = (3/4) (2)^{n â€“ 1}

(384 Ã— 4)/3 = (2)^{n â€“ 1}

(1536)/3 = (2)^{n â€“ 1}

** Â ** 512 = 2^{n â€“ 1}

2^{9} = 2^{n â€“ 1}

By comparing both left hand side and right hand side,

9 = n â€“ 1

n = 9 + 1

n = 10

The number of terms of a G.P. is 10.

**6. Find the value of x such that,**

**(i) -2/7, x, -7/2 are three consecutive terms of a G.P.**

**Solution:-**

From the question,

x^{2} = -2/7 Ã— -7/2

x^{2} = 1

x = Â± 1

Therefore, x = 1 or x = – 1

**(ii) x + 9, x â€“ 6 and 4 are three consecutive terms of a G.P.**

**Solution:-**

From the question,

(x – 6)^{2} = (x + 9) Ã— 4

x^{2} â€“ 12x + 36 = 4x + 36

x^{2} â€“ 12x â€“ 4x + 36 â€“ 36 = 0

x^{2} â€“ 16x = 0

x(x â€“ 16) = 0

Either let us take x â€“ 16 = 16

Or x = 0

So, x = 0, 16

**(iii) x, x + 3, x + 9 are first three terms of a G.P. Find the value of x.**

**Solution:-**

From the question,

(x + 3)^{2} = x(x + 9)

x^{2} + 6x + 9 = x^{2} + 9x

9 = 9x â€“ 6x

9 = 3x

X = 9/3

X = 3

**7. If the fourth, seventh and tenth terms of a G.P. are x, y, z respectively, prove that x, y, z are in G.P.**

**Solution:-**

From the question it is given that,

a_{4} = x

a_{7} = y

a_{10} = z

Now we have to prove that, x, y, z are in G.P.

Then, by the formula a_{n} = ar^{n â€“ 1 }

a_{4} = ar^{4 â€“ 1}

a_{4} = a^{3}

a_{4} = x

So, a_{7} = a^{7 â€“ 1}

a_{7} = a^{6}

a_{7} = y

a_{10} = a^{10 -1 }

a_{10} = a^{9}

a_{10} = z

x, y, z are in G.P. then,

y^{2} = xz

Then, xz = ar^{3} Ã— ar^{9}

= a^{2}r^{3} ^{+ 9}

= a^{2}r^{12}

y^{2} = (ar^{6})^{2}

y^{2} = a^{2}r^{12}

By comparing left hand side and right hand side

LHS = RHS

Therefore, x, y and z are in G.P.

**8. The 5 ^{th}, 8^{th} and 11^{th} terms of a G.P. are p, q and s respectively. Show that qÂ² = ps.**

**Solution:-**

From the question it is given that,

a_{5} = p

a_{8} = q

a_{11} = s

Now we have to prove that, qÂ² = ps

Then, by the formula a_{n} = ar^{n â€“ 1 }

a_{5} = ar^{5 â€“ 1}

a_{5} = a^{4}

a_{5} = p

So, a_{8} = a^{8 â€“ 1}

a_{8} = a^{7}

a_{8} = q

a_{11} = a^{11 -1 }

a_{11} = a^{10}

a_{11} = s

p, q, s are in G.P. then,

q^{2} = (ar^{7})^{2}

= ar^{14}

Then, px = ar^{4} Ã— ar^{10}

= a^{2}r^{4} ^{+ 10}

= a^{2}r^{14}

Therefore, q^{2} = ps

**9. If a, a ^{2}+ 2 and a^{3}Â + 10 are in G.P., then find the values(s) of a.**

**Solution:-**

From the question,

(a^{2} + 2)^{2} = a(a^{3} + 10)

a^{4} + 4 = a^{4} + 10a

4a^{2} â€“ 10a + 4 = 0

2a^{2} â€“ 5a + 2 = 0

2a^{2} â€“ a â€“ 4a + 2 = 0

a(2a – 1) â€“ 2(2a – 1) = 0

(2a – 1) (a – 2) = 0

Then, 2a â€“ 1 = 0

a = Â½

a â€“ 2 = 0

a = 2

Therefore, a = 2 or a= Â½

**10. Find the geometric progression whose 4 ^{th} term is 54 and the 7^{th} term is 1458.**

**Solution:-**

From the question it is given that,

4^{th} term a_{4} = 54

7^{th} term a_{7} = 1458

ar^{3} = 54

ar^{6} = 1458

Now dividing we get,

ar^{6}/ar^{3} = (1458/54)

r^{6 â€“ 3} = 27

r^{3} = 3^{3}

r = 3

Then, ar^{3} = 54

a Ã— 27 = 54

a = 54/27

a = 2

Therefore G.P. is 2, 6, 18, 54, â€¦

**11. The sum of first three terms of a G.P. is 39/10 and their product is 1. Find the common ratio and the terms.**

**Solution:-**

From the question it is give that,

The sum of first three terms of a G.P. is 39/10

The product of first three terms of a G.P. is 1

Let us assume that a be the first term and â€˜râ€™ be the common ratio,

And also assume that, three terms of the G.P. is a/r, a, ar,

The sum of three terms = (a/r) + a + ar = 39/10

Take out â€˜aâ€™ as common then, we get

a(1/r + 1 + r) = 39/10 â€¦ [equation (i)]

Now, product of three terms = (a/r) Ã— a Ã— ar = 1

a^{3}r/r = 1

a^{3}= 1

a^{3} = 1^{3}

a = 1

Substitute the value of â€˜aâ€™ in equation (i),

1(1/r + 1 + r) = 39/10

(1 + r + r^{2})/r = 39/10

By cross multiplication we get,

10(1 + r + r^{2})/r = 39r

10 + 10r + 10r^{2} = 39r

Transposing 39r from right hand side to left hand side it becomes â€“ 39r,

10 + 10r + 10r^{2} â€“ 39r = 0

10r^{2} â€“ 29r + 10 = 0

10r^{2} â€“ 25r â€“ 4r + 10 = 0

5r(2r – 5) â€“ 2(2r – 5) = 0

(2r – 5) (5r – 2) = 0

So, 2r â€“ 5 = 0

r = 5/2

5r â€“ 2 = 0

r = 2/5

Therefore, r = 5/2 or 2/5

Then the terms if r = 5/2 are, 1, 5/2, 25/4, â€¦

The terms if r = 2/5 are, 1, 2/5, 4/25, â€¦

**12. Three numbers are in A.P. and their sum is 15. If 1, 4 and 19 are added to these numbers respectively, the resulting numbers are in G.P. Find the numbers.**

**Solution:-**

From the question it is give that,

The sum of first three terms of a A.P. is 15

Let us assume three numbers are a â€“ d, a, a + d.

The sum of three terms = a â€“ d + a + a + d = 15

a = 15/3

a = 5

Then, adding 1, 4, 19 in the terms

The numbers become, a â€“ d + 1, a + 4, a + d + 19

Therefore, b^{2} = ac

(a + 4)^{2} = (a â€“ d + 1) (a + d + 19)

Simplify the above terms,

a^{2} + 8a + 16 = a^{2} + ad + 19a â€“ ad â€“ a^{2} â€“ 19d + a + d + 19

a^{2} + 8a + 16 = a^{2} â€“ d^{2} â€“ 18d + 20a + 19

8a + 16 = 20a â€“ 18d â€“ d^{2} + 19

8a + 16 â€“ 20a + 18d + d^{2} â€“ 19 = 0

d^{2} + 18d â€“ 12a â€“ 3 = 0

d^{2} + 18d â€“ (12 Ã— 5) â€“ 3 = 0

d^{2} + 18d â€“ 60 â€“ 3= 0

d^{2} + 18d â€“ 63 = 0

d^{2} + 21d â€“ 3d â€“ 63 = 0

d(d + 21) â€“ 3(d + 21) = 0

(d + 21) (d – 3) = 0

So, d + 21 = 0

d = – 21

d â€“ 3 = 0

d = 3

Then the terms if d = 3 and a = 5,

Then G.P. 5 â€“ 3 = 2, 5, 5 + 3 = 8

The terms if d = – 21 are 5 â€“ (-21) = 5 + 21 = 26, 5, 5 â€“ 21 = – 16

Exercise 9.5

**1. Find the sum of:**

**(i) 20 terms of the series 2 + 6 + 18 + â€¦**

**Solution:-**

From the question,

First term a = 2,

Common ratio r = 6/2 = 3

Number of terms n = 20

So, S_{20} = a(r^{n} – 1)/r â€“ 1

= 2(3^{20} – 1)/3 â€“ 1

= 2(3^{20} – 1)/2

= 3^{20} â€“ 1

Therefore, S_{20} = 3^{20} – 1

(ii) 10 terms of series 1 + âˆš3 + 3 + â€¦

**Solution:-**

From the question,

First term a = 1,

Common ratio r = âˆš3/1 = âˆš3

Number of terms n = 10

So, S_{10} = a(r^{n} – 1)/r â€“ 1

= 1((âˆš3)^{10} – 1)/ âˆš3 â€“ 1

Multiplying (âˆš3 + 1) for both numerator and denominator we get,

= ((âˆš3^{10} – 1) (âˆš3 + 1))/ ((âˆš3 â€“ 1) (âˆš3 + 1)

= (3^{5} – 1) (âˆš3 + 1))/3 â€“ 1 â€¦ [by rationalizing the denominator]

= ((243 – 1)( âˆš3 + 1))/2

= 242(âˆš3 + 1)/2

= 121(âˆš3 + 1)

Therefore, S_{10} = 121(âˆš3 + 1

**(iii) 6 terms of the GP 1, -2/3, 4/9, â€¦**

**Solution:-**

From the question,

First term a = 1,

Common ratio r = -2/3 Ã— 1= -2/3

Number of terms n = 6

So, S_{6} = a(r^{n} – 1)/r â€“ 1

= 1[1 – (-2/3)^{6}]/(1 + (2/3))

= (3/5) (1 â€“ (-2^{6}/3^{6}))

= (3/5) (1 â€“ (64/729))

= (3/5) ((729 – 64)/729)

= 3/5 Ã— (665/729)

= 133/243

**(iv) 5 terms and n terms of the series 1 + 2/3 + 4/9 + â€¦**

**Solution:-**

From the question,

First term a = 1,

Common ratio r = 2/3 Ã— 1= -/3

Number of terms n = 5

So, S_{n} = a(1 – r^{n})/1 â€“ r

= 1[1 â€“ (2/3)^{n}]/(1 â€“ 2/3)

S_{n }= 3[1 â€“ (2/3)^{n}]

Then, S_{5} = 3[1 â€“ (2/3)^{5}]

= 3[1 â€“ (32/243)]

= 3((243 â€“ 32)/243)

= 211/81

**2. Find the sum of the series 81 â€“ 27 + 9 â€¦ – 1/27**

**Solution:-**

From the question it is given that,

First term a = 81

r = -27/81

= -1/3

Last term l = -1/27

S_{n} = (a â€“ lr)/(l – r)

= [81 + ((1/27) Ã— (-1/3)]/[1 + (1/3)]

= [(81 â€“ (1/81))]/(4/3)

= (6561 – 1)/[81 Ã— (4/3)]

= (6560 Ã— 3)/(81 Ã— 4)

= 1640/27

**3. The n ^{th} term of a G.P. is 128 and the sum of its n terms is 255. If its common ratio is 2, then find its first term.**

**Solution:-**

From the question it is given that,

The n^{th} term of a G.P. T_{n} = 128

The sum of its n terms S_{n} = 255

Common ratio r = 2

We know that, T_{n} = ar^{n â€“ 1}

128 = a2^{n â€“ 1}

a = 128/2^{n â€“ 1} â€¦ [equation (i)]

Also we know that, S_{n} = a(r^{n} – 1)/(r – 1)

255 = a(2^{n} – 1)/(2 – 1)

By cross multiplication we get,

255 = a(2^{n} – 1)

a = 255/(2^{n}Â â€“ 1) â€¦ [equation (ii)]

Now, consider both the equation(i) and equation (ii)

255/(2^{n} – 1) = 128/(2^{n – 1})

By cross multiplication we get,

255 Ã— 2^{n â€“ 1} = 128(2^{n}Â – 1)

255 Ã— 2^{n â€“ 1 }= 128 Ã— 2^{n} â€“ 128

(255 Ã— 2^{n})/2 = 128 Ã— 2^{n} â€“ 128

255 Ã— 2^{n} = 256 Ã— 2^{n} â€“ 256

256 Ã— 2^{n} â€“ 255 Ã— 2^{n} = 256

By simplification,

2^{n}Â = 256

2^{n}Â = 2^{8}

By comparing both LHS and RHS, we get,

Then, 128 = a2^{7}

128 = a Ã— 128

a = 128/128

a = 1

Therefore, the value of a is 1.

**4. (i) How many terms of the G.P. 3, 3 ^{2}, 3^{3}, â€¦ are needed to give the sum 120?**

**Solution:-**

From the question it is given that,

Terms of the G.P. 3, 3^{2}, 3^{3}, â€¦

Sum of the terms = 120

The first term a = 3

r = 3^{2}/3

= 9/3

= 3

We know that, S_{n} = a(r^{n} – 1)/r â€“ 1 = 120

3(3^{n} – 1)/(3 – 1) = 120

3(3^{n} – 1)/2 = 120

By cross multiplication we get,

3^{n}Â â€“ 1 = (120 Ã—2)/3

3^{n} â€“ 1 = 240/3

3^{n} â€“ 1 = 80

3^{n} = 80 +1

3^{n} = 81

3^{n} = 3^{4}

Therefore, n = 4

**(ii) How many terms of the G.P. 1, 4, 16, â€¦ must be taken to have their sum equal to 341?**

**Solution:-**

From the question it is given that,

Terms of the G.P. 1, 4, 16, â€¦

Sum of the terms = 341

The first term a = 1

r = 4/1

= 4

We know that, S_{n} = a(r^{n} – 1)/r â€“ 1 = 341

1(4^{n} – 1)/(4 – 1) = 341

1(4^{n} – 1)/3 = 341

By cross multiplication we get,

4^{n}Â â€“ 1 = (341 Ã— 3)

4^{n} â€“ 1 = 1023

4^{n} = 1023 + 1

4^{n} = 1024

4^{n} = 4^{5}

Therefore, n = 5

**5. How many terms of the 2/9 â€“ 1/3 + Â½ + â€¦ will make the sum 55/72?**

**Solution:-**

From the question it is given that,

Terms of G.P. is 2/9 â€“ 1/3 + Â½ + â€¦

Sum of the terms = 55/72

The first term a = 2/9

r = -1/3 Ã· 2/9 = (-1/3) Ã— (9/2) = – 3/2

We know that, S_{n} = a(r^{n} – 1)/r â€“ 1 = 55/72

^{n})]/(1 + (3/2)) = 55/72

1 â€“ (-3/2)^{n} = (55/72) Ã— (5/2) Ã— (9/2)

(1 â€“ (-1))^{n} (3/2)^{n} = 275/32

1 + 1(3/2)^{n} = 275/32

(3/2)^{n} = 275/32 â€“ 1

(3/2)^{n} = (275 – 32)/32

(3/2)^{n} = 243/32

(3/2)^{n} = (3/2)^{5}

Therefore, n = 5

**6. The 2 ^{nd} and 5^{th} terms of a geometric series are -Â½ and sum 1/16 respectively. Find the sum of the series up to 8 terms.**

**Solution:-**

From the question it is given that,

a_{2} = -Â½

a_{5} = 1/16

We know that, a_{2} = ar^{n â€“ 1}

= ar^{2 â€“ 1}

a_{2} = ar = -Â½ â€¦ [equation (i)]

a_{5} = ar^{5 â€“ 1}

= ar^{4}

a_{5} = ar^{4} = 1/16 â€¦ [equation (ii)]

Now, dividing equation (ii) by (i) we get,

r^{3} = 1/16 Ã· (-Â½)

= (1/16) Ã— (-2)

= -1/8

r^{3} = (-1/2)^{3}

So, r = -Â½

ar = -Â½

a Ã— (-Â½) = -Â½

a = – Â½ Ã— (-2/1)

a = 1

Therefore, a = 1 and r = -Â½

Then, S_{8} = a(1 â€“ r^{n})/(1 – r)

= 1[1 – (-1/2)^{8}]/(1 + Â½)

= [1 â€“ (1/256)]/(3/2)

= (255/256) Ã— (2/3)

= (510/768)

= 85/128

**7. The first term of G.P. is 27 and 8 ^{th} term is 1/81. Find the sum of its first 10 terms.**

**Solution:-**

From the question it is given that,

First term a = 27

8^{th} term a_{8} = 1/81

Then, a_{n} = ar^{n â€“ 1}

a_{8} = ar^{8 â€“ 1} = 1/81

a_{8} = ar^{7} = 1/81

ar^{7} = 1/81

27r^{7} = 1/81

r^{7} = 1/(81 Ã— 27)

r^{7} = 1/2187

r^{7} = 1/(3^{7})

r = 1/3

So, S_{10} = a(1 â€“ r^{n})/(1 – r)

= 27[1 – (1/3)^{10}]/(1 â€“ 1/3)

= 27[1 â€“ (1/3^{10})]/((3 â€“ 1)/3)

= ((27 Ã— 3)/2) [1 â€“ 1/3^{10}]

= (81/2) [1 â€“ 1/3^{10}]

**8. Find the first term of the G.P. whose common ratio is 3, last term is 486 and the sum of whose terms is 728.**

**Solution:-**

From the question it is given that,

Common ratio r = 3

Last term = 486

Sum of the terms = 728

We know that, S_{n} = a(r^{n} – 1)/(r – 1)

= a(3^{n} – 1)/(3 â€“ 1) = 728

a(3^{n} – 1)/2 = 728

a(3^{n} – 1) = 728 Ã— 2

a(3^{n} – 1) = 1456 â€¦ [equation (i)]

Then, last term = ar^{n â€“ 1}

486 = a Ã— 3^{n â€“ 1}

486 = a(3^{n}/3)

486 Ã— 3 = a3^{n}

1458 = a3^{n} â€¦ [equation (ii)]

Consider equation (i), a(3^{n} – 1) = 1456

a3^{n} â€“ a = 1456

Substitute the value of a3^{n} in equation (i),

1458 â€“ a = 1456

a = 1458 â€“ 1456

a = 2

Therefore, the first term a is 2.

**9. In a G.P. the first term is 7, the last term is 448, and the sum is 889. Find the common ratio.**

**Solution:-**

From the question it is given that,

First term a is = 7

Then, last term is = 448

Sum = 889

We know that, last term = ar^{n â€“ 1}

7r^{n -1} = 448

r^{n â€“ 1} = 448/7

r^{n â€“ 1} = 64 â€¦ [equation (i)]

So, sum = a(r^{n} – 1)/(r â€“ 1) = 889

7(r^{n} – 1)/(r – 1) = 889

(r^{n} – 1)/(r – 1) = 889/7

(r^{n} – 1)/(r – 1) = 127 â€¦ [equation (ii)]

Consider the equation (i),

r^{n}/r = 64

r^{n} = 64r

Now substitute the value of r^{n} in equation (ii),

(64r â€“ 1)/(r – 1) = 127

64r â€“ 1 = 127r â€“ 127

127r â€“ 64r = -1 + 127

63r = 126

r = 126/63

r = 2

Therefore, common ratio = 2

**10. Find the third term of a G.P. whose common ratio is 3 and the sum of whose first seven terms is 2186.**

**Solution:-**

From the question it is given that,

Common ratio r = 3

S_{7} = 2186

We know that, S_{n} = a(r^{n} – 1)/(r – 1)

S_{7} = a(3^{n} – 1)/(3 – 1)

2186 = a(3^{n}Â – 1)/2

By cross multiplication,

(2186 Ã— 2) = a(3^{7} – 1)

(4372) = a(2187 – 1)

4372 = a2186

a = 4372/2186

a = 2

Then, a_{3} = ar^{3 â€“ 1}

= ar^{2}

= 2 Ã— 3^{2}

= 2 Ã— 9

a_{3} = 18

**11. If the first term of a G.P. is 5 and the sum of first three terms isÂ 31/5, find the common ratio.**

**Solution:-**

From the question it is given that,

First term of a G.P. is a = 5

The sum of first three terms isÂ S_{3} = 31/5

We know that, S_{n} = a(r^{n} – 1)/(r – 1)

S_{3} = a(r^{3} – 1)/(r – 1)

31/5 = 5(r^{3} – 1)/(r – 1)

31/(5 Ã— 5) = (r^{3} – 1)/(r – 1)

31/25 = (r^{3} – 1)/(r – 1)

(r – 1) (r^{2} + r + 1)/(r – 1) = 31/25

r^{2} + r + 1 = 31/25

By cross multiplication we get,

25(r^{2} + r + 1) = 31

25r^{2} + 25r + 25 = 31

Transposing 31 from right hand side to left hand side it becomes â€“ 31,

25r^{2} + 25r + 25 â€“ 31 =

25r^{2} + 25r â€“ 6 = 0

25r^{2} + 30r â€“ 5r â€“ 6 = 0

5r(5r + 6) â€“ 1(5r + 6) = 0

(5r – 1) (5r + 6) = 0

Take 5r â€“ 1 = 0

r = 1/5

or 5r + 6 = 0

r = -6/5

Therefore, common ratio r = 1/5 or -6/5.

Chapter – test

**1. Write the first four terms of the A.P. when its first term is â€“ 5 and the common difference is â€“ 3.**

**Solution:-**

From the question it is given that,

First term a = – 5

Common difference d = -3

Then the first four terms are = – 5 + (-3) = -5 – 3 = – 8

-8 + (-3) = – 8 – 3 = -11

– 11 + (-3) = – 11 – 3 = – 14

Therefore, first four terms are -5, -8, -11 and -14.

**2. Verify that each of the following lists of numbers is an A.P., and the write its next three terms:**

**(i) 0, Â¼, Â½, Â¾, â€¦ **

**Solution:- **

From the question it is given that,

First term a = 0

Common difference = Â¼ – 0 = Â¼

So, next three numbers are Â¾ + Â¼ = 4/4 = 1

1 + Â¼ = (4 + 1)/4 = 5/4

5/4 + Â¼ = 6/4 = 3/2

Therefore, the next three term are 1, 5/4 and 3/2.

**(ii) 5, 14/3, 13/3, 4, â€¦**

**Solution:-**

From the question it is given that,

First term a = 5

Common difference = 14/3 – 5 = (14 – 15)/3 = -1/3

So, next three numbers are 4 + (-1/3) = (12 – 1)/3 = 11/3

11/3 + (-1/3) = (11 – 1)/3 = 10/3

10/3 + (-1/3) = (10 – 1)/3 = 9/3 = 3

Therefore, the next three term are 11/3, 10/3 and 3.

**3. The n ^{th} term of an A.P. is 6n + 2. Find the common difference.**

**Solution:-**

From the question it is given that,

n^{th} term is 6n + 2

So, T_{n} = 6n + 2

Now, we start giving values, 1, 2, 3, â€¦ in the place of n, we get,

T_{1} = (6 Ã— 1) + 2 = 6 + 2 = 8

T_{2} = (6 Ã— 2) + 2 = 12 + 2 = 14

T_{3} = (6 Ã— 3) + 2 = 18 + 2 = 20

T_{4} = (6 Ã— 4) + 2 = 24 + 2 = 26

Therefore, A.P. is 8, 14, 20, 26, â€¦

So, common difference d = 14 â€“ 8 = 6

**4. Show that the list of numbers 9, 12, 15, 18, â€¦ form an A.P. Find its 16 ^{th} term and the n^{th}.**

**Solution:-**

From the question,

The first term a = 9

Then, difference d = 12 â€“ 9 = 3

15 â€“ 12 = 3

18 â€“ 15 = 3

Therefore, common difference d = 3

From the formula, a_{n} = a + (n – 1)d

T_{n} = a + (n – 1)d

= 9 + (n – 1)3

= 9 + 3n – 3

= 6 + 3n

So, T_{16} = a + (n – 1)d

= 9 + (16 – 1)3

= 9 + (15)(3)

= 9 + 45

= 54

**5. Find the 6 ^{th} term from the end of the A.P. 17, 14, 11, â€¦, â€“ 40.**

**Solution:-**

From the question it is given that,

First term a = 17

Common difference = 14 â€“ 17 = – 3

Last term l = – 40

L = a + (n – 1)d

-40 = 17 + (n – 1)(-3)

-40 â€“ 17 = -3n + 3

– 57 – 3 = -3n

n = -60/-3

n = 20

Therefore, 6^{th} term form the end = l â€“ (n – 1)d

= – 40 â€“ (6 – 1)(-3)

= – 40 â€“ (5)(-3)

= – 40 + 15

= – 25

**6. If the 8 ^{th} term of an A.P. is 31 and the 15^{th} term is 16 more than its 11^{th} term, then find the A.P.**

**Solution:-**

From the question it is given that,

a_{8} = 31

a_{15} = the 15^{th} term is 16 more than its 11^{th} term = a_{11} + 16

we know that, a_{n} = a + (n – 1)d

So, a_{8} = a + 7d = 31 â€¦ [equation (i)]

a_{15} = a + 14d = a + 10d + 16

14d â€“ 10d = 16

4d = 16

d = 16/4

d = 4

Now substitute the value of d in equation (i) we get,

a + (7 Ã— 4) = 31

a + 28 = 31

a = 31 â€“ 28

a = 3

So, 3 + 4 = 7, 7 + 4 = 11, 11 + 4 = 15

Therefore, A.P. is 3, 7, 11, 15, â€¦

**7. The 17 ^{th} term of an A.P. is 5 more than twice its 8^{th} term. If the 11^{th} term of the A.P. is 43, then find the w^{th} term.**

**Solution:-**

From the question it is given that,

a_{17} = 5 more than twice its 8^{th} term = 2a_{8} + 5

a_{11} = 43

a_{n} = ?

We know that, a_{11} = a + 10d = 43 â€¦ [equation (i)]

a_{17} = 2a_{8} + 5

a + 16d = 2(a + 7d) + 5

a + 16d = 2a + 14d + 5

2a â€“ a = 16d â€“ 14d â€“ 5

a = 2d â€“ 5 â€¦ [equation (ii)]

Now substitute the value of a in equation (i) we get,

2d â€“ 5 + 10d = 43

12d = 43 + 5

12d = 48

d = 48/12

d = 4

To find out the value of a substitute the value of d in equation (i)

a + (10 Ã— 4) = 43

a + 40 = 43

a = 43 â€“ 40

a = 3

Then, a_{n} = a + (n – 1)d

= 3 + 4(n – 1)

= 3 + 4n â€“ 4

= 4n â€“ 1

**8. The 19 ^{th} term of an A.P. is equal to three times its 6^{th} term. If its 9^{th} term is 19, find the A.P.**

**Solution:-**

From the question it is given that,

a_{19} = 19^{th} term of an A.P. is equal to three times its 6^{th} term = 3a_{6}

a_{9} = 19

As we know, a_{n} = a + (n – 1)d

a_{9} = a + 8d = 19 â€¦ [equation (i)]

Then, a_{19} = 3(a + 5d)

a + 18d = 3a + 15d

3a â€“ a = 18d â€“ 15d

2a = 3d

a = (3/2)d

Now substitute the value of a in equation (i) we get,

** **(3/2)d + 8d = 19

(3d + 16d)/2 = 19

(19/2)d = 19

d = (19 Ã— 2)/19

d = 2

To find out the value of a substitute the value of d in equation (i)

a + 8d = 19

a + (8 Ã— 2) = 19

a + 16 = 19

a = 19 â€“ 16

a = 3

Therefore, A.P. is 3, 5, 7, 9, â€¦

**9. If the 3 ^{rd} and the 9^{th} terms of an A.P. are 4 and â€“ 8 respectively, then which term of this A.P. is zero?**

**Solution:-**

From the question it is given that,

a_{3} = 4

a_{9} = – 8

We know that, a_{3} = a + 2d = 4 â€¦ [equation (i)]

a_{9} = a + 8d = -8 â€¦ [equation (ii)]

Now, subtracting equation (i) from equation (ii)

(a + 8d) â€“ (a + 2d) = -8 â€“ 4

a + 8d â€“ a â€“ 2d = -12

6d = -12

d = -12/6

d = -2

To find out the value of a substitute the value of d in equation (i)

a + 2d = 4

a + (2 Ã— (-2)) = 4

a â€“ 4 = 4

a = 4 + 4

a = 8

let us assume n^{th} term be zero, then

a + (n – 1)d = 0

8 + (n – 1)(-2) = 0

-2n + 2 = -8

-2n = -8 â€“ 2

-2n = -10

n = -10/-2

n = 5

Therefore, 0 will be the fifth term.

**10. Which term of the list of numbers 5, 2, â€“ 1, â€“ 4, â€¦ is â€“ 55?**

**Solution:-**

From the question it is given that,

First term a = 5

n^{th} term = -55

Common difference d = 2 â€“ 5 = – 3

We know that, a_{n} = a + (n – 1)d

– 55 = 5 + (n – 1)(-3)

-55 â€“ 5 = – 3n + 3

-60 â€“ 3 = -3n

-63 = -3n

n = -63/-3

n = 21

Therefore, -55 is the 21^{st} term.

**11. The 24 ^{th} term of an A.P. is twice its 10^{th} term. Show that its 72^{nd} term is four times its 15^{th} term.**

**Solution:-**

From the question it is given that,

The 24^{th} term of an A.P. is twice its 10^{th} term = a_{24} = 2a_{10}

We have to show that, 72^{nd} term is four times its 15^{th} term = a_{72} = 4a_{15}

We know that, a_{24} = a + 23d = 2a_{10}

a + 23d = 2(a + 9d)

a + 23d = 2a + 18d

2a – a = 23d â€“ 18d

a = 5d â€¦ [equation (i)]

a_{72} = 4a_{15}

a + 71d = 4(a + 14d)

Substitute the value of a we get,

5d + 71d = 4(5d + 14d)

76d = 4(19d)

Therefore, it is proved that 72^{nd} term is four times its 15^{th} term.

**12. Which term of the list of numbers 20, 19Â¼, 18Â½, 17Â¾, â€¦ is the first negative term?**

**Solution:-**

From the question it is given that,

First term a = 20

Common difference d = 19Â¼ – 20 = 77/4 â€“ 20 = (77 â€“ 80)/4 = -Â¾

We know that, a_{n} = a + (n – 1)d

a_{n} = 20 + (n â€“ 1) (-Â¾)

a_{n} = 20 â€“ Â¾n + Â¾

a_{n} = 20 + Â¾ – Â¾n

a_{n} = (80 + 3)/4 â€“ Â¾n

a_{n} = 83/4 â€“ Â¾n < 0

83/4 < Â¾n

83 < 3n

83/3 < n

28 < n

Therefore, 28^{th} is the first negative term.

**13. How many three digit numbers are divisible by 9?**

**Solution:-**

The three digits numbers which are divisible by 9 are 108, 117, 126, â€¦, 999

Then, first term a = 108

Common difference = 9

Last term = 999

We know that, l = a_{n} = a + (n – 1)d

999 = 108 + (n – 1)9

999 â€“ 108 = 9n â€“ 9

891 + 9 = 9n

900 = 9n

n = 900/9

n = 100

Therefore, there are 100 three digits numbers.

**14. The sum of three numbers in A.P. is â€“ 3 and the product is 8. Find the numbers.**

**Solution:-**

From the question it is given that,

The sum of three numbers in A.P. = â€“ 3

The product of three numbers in A.P. = 8

Let us assume the 3 numbers which are in A.P. are, a â€“ d, a, a + d

Now adding 3 numbers = a â€“ d + a + a + d = – 3

3a = -3

a = -3/3

a = -1

From the question, product of 3 numbers is â€“ 35

So, (a – d) Ã— (a) Ã— (a + d) = 8

a(a^{2} â€“ d^{2}) = 8

-1 ((-1)^{2} â€“ d^{2} = 8

1 – d^{2} = 8/-1

1 – d^{2} = -8

d^{2} = 8 + 1

d^{2}= 9

d = âˆš9

d = Â±3

Therefore, the numbers are if d = 3 (a – d) = – 1 â€“ 3 = – 4

a = -1

(a + d) = – 1 + 3 = 2

If d = – 6

The numbers are (a – d) = – 1 â€“ (-3) = – 1 + 3 = 2

a = -1

(a + d) = -1 + (-3) = -1 â€“ 3 = -4

Therefore, the numbers -4, -1, 2,â€¦ and 2, -1, -4,â€¦ are in A.P.

**15. The angles of a quadrilateral are in A.P. If the greatest angle is double of the smallest angle, find all the four angles.**

**Solution:-**

From the question it is given that,

The angles of a quadrilateral are in A.P.

Greatest angle is double of the smallest angle

Let us assume the greatest angle of the quadrilateral is a + 3d,

Then, the other angles are a + d, a â€“ d, a â€“ 3d

So, a â€“ 3d is the smallest

Therefore, a + 3d = 2(a â€“ 3d)

a + 3d = 2a â€“ 6d

6d + 3d = 2a â€“ a

9d = a â€¦ [equation (i)]

We know that the sum of all angles of quadrilateral is 360^{o}.

a â€“ 3d + a â€“ d + a + d + a+ 3d = 360^{o}

4a = 360^{o}

a = 360/4

a = 90^{o}

Now, substitute the value of a in equation (i) we get,

9d = 90

d = 90/9

d = 10

Substitute the value of a and d in assumed angles,

Greatest angle = a + 3d = 90 + (3 Ã— 10) = 90 + 30 = 120^{o}

Then, other angles are = a + d = 90^{o} + 10^{o} = 100^{o}

a â€“ d = 90^{o} â€“ 10^{o}Â = 80^{o}

a â€“ 3d = 90^{o}Â â€“ (3 Ã— 10) = 90 â€“ 30 = 60^{o}

Therefore, the angles of quadrilateral are 120^{o}, 100^{o}, 80^{o} and 60^{o}.

**16. Find the sum of first 20 terms of an A.P. whose n ^{th} term is 15 â€“ 4n.**

**Solution:-**

From the question it is given that,

n^{th} term is 15 â€“ 4n

So, a_{n} = 15 â€“ 4n

Now, we start giving values, 1, 2, 3, â€¦ in the place of n, we get,

a_{1} = 15 â€“ (4 Ã— 1) = 15 â€“ 4 = 11

a_{2} = 15 â€“ (4 Ã— 2) = 15 â€“ 8 = 7

a_{3 }= 15 â€“ (4 Ã— 3) = 15 â€“ 12 = 3

a_{4} = 15 â€“ (4 Ã— 4) = 15 â€“ 16 = – 1

Then, a_{20} = 15 â€“ (4 Ã— 20) = 15 â€“ 80 = -65

So, 11, 7, 3, -1, â€¦ -65 are in A.P.

Therefore, first term a = 11

Common difference = -4

n = 20

S_{20} = (n/2) [2a + (n – 1)d]

= (20/2) [(2 Ã— 11) + (20 – 1)(-4)]

= 10 [22 â€“ (19) (-4)]

= 10 [22 â€“ 76]

= 10(-54)

= -540

Therefore, the sum of first 20 terms of an A.P. is -540.

**17. Find the sum : 18 + 15Â½ + 13 + â€¦ + (-49Â½)**

**Solution:-**

From the question it is given that,

First term a = 18

Common difference d = 15Â½ – 18

= 31/2 â€“ 18

= (31 â€“ 36)/2

= -5/2

Last term = -49Â½ = -99/2

We know that, a_{n} = a + (n – 1)d

-99/2 = 18 + (n – 1)(-5/2)

(-99/2) â€“ (18/1) = (n – 1)(-5/2)

(-99 – 36)/2 = (-5/2)(n – 1)

(-135/2) = (-5/2) (n – 1)

(-135/2) Ã— (-2/5) = n â€“ 1

-135/-5 = n â€“ 1

27 = n â€“ 1

n = 27 + 1

n = 28

Then, S_{n} = (n/2) [2a + (n – 1)d]

S_{28} = (28/2) [(2 Ã— 18) + (28 – 1)(-5/2)]

S_{28} = 14[36 + (27 Ã— (-5/2))]

S_{28} = 14[36 â€“ (135/2)]

S_{28} = 14 [(72 – 135)/2]

S_{28} = 14 (-63/2)

S_{28} = – 441

**18. (i) How many terms of the A.P. â€“ 6, (-11/2), â€“ 5, â€¦ make the sum â€“ 25?**

**Solution:-**

From the question it is given that,

Terms of the A.P. is -6, (-11/2) â€“ 5, â€¦

The first term a = -6

Common difference d = (-11/2) â€“ (-6)

= (-11/2) + 6

= (-11 + 12)/2

= Â½

The terms are make the sum â€“ 25

Then, S_{n} = (n/2)(2a + (n â€“ 1)d)

-25 = (n/2) [(2 Ã— (-6)) + (n – 1) (Â½)]

(-25 Ã— 2) = n [-12 + Â½n â€“ Â½]

-50 = n [(-25/2) + (Â½n)]

Â½n^{2} â€“ (25/2)n + 50 = 0

n^{2} â€“ 25n + 100 = 0

n^{2} â€“ 5n â€“ 20n + 100 = 0

n(n – 5) â€“ 20(n – 5) = 0

(n – 5) (n – 20) = 0

So, n â€“ 5 = 0

n = 5

or n â€“ 20 = 0

n = 20

Therefore, number of terms are 5 or 20.

**(ii) Solve the equation 2 + 5 + 8 + â€¦ + x = 155**

**Solution:-**

From the question it is given that,

First term a = 2

Last term = x

Common difference d = 5 â€“ 2 = 3

Then, sum of the terms = 155

L = a + (n – 1)d

x = 2 + (n – 1)3

x = 2 + 3n â€“ 3

x = 3n â€“ 1 â€¦ [equation (i)]

We know that, S_{n} = (n/2) [2a + (n – 1)d]

155 = (n/2) [(2 Ã— 2) + (n – 1) Ã— 3]

155 Ã— 2 = n[4 + 3n – 3]

310 = n(3n + 1)

310 = 3n^{2} + n

3n^{2} + n â€“ 310 = 0

3n^{2} â€“ 30n + 31n â€“ 310 = 0

3n(n – 10) + 31(n – 10) = 0

(n – 10) (3n + 31) = 0

So, n â€“ 10 = 0

n = 10

or 3n + 31 = 0

n = -31/3

negative is not possible.

Therefore, n = 10

Now, substitute the value of n in equation (i),

x = 3n â€“ 1

= (3 Ã— 10) â€“ 1

= 30 â€“ 1

= 29

**19. If the third term of an A.P. is 5 and the ratio of its 6 ^{th} term to the 10^{th} term is 7 : 13, then find the sum of first 20 terms of this A.P.**

**Solution:-**

From the question it is given that,

The third term of an A.P. a_{3} = 5

The ratio of its 6^{th} term to the 10^{th} term a_{6} : a_{10} = 7 : 13

We know that, a_{n} = a + (n – 1)d

a_{3} = a + (3 – 1)d = 5

= a + 2d = 5 â€¦ [equation (i)]

Then, a_{6}/a_{10} = 7/13

(a + 5d)/(a + 9d) = 7/13

By cross multiplication we get,

13(a + 5d) = 7(a + 9d)

13a + 75d = 7a + 63d

13a â€“ 7a + 65d â€“ 63d = 0

6a + 2d = 0

Divide by 2 on both side we get,

3a + d = 0

d = -3a â€¦ [equation (ii)]

Substitute the value of d in equation (i),

a + 2(-3a) = 5

a â€“ 6a = 5

-5a = 5

a = -5/5

a = -1

Now substitute the value of a in equation (ii),

d = -3(-1)

d = 3

Then, sum of first 20 terms,

= (n/2) [2a + (n – 1)d]

= (20/2)[(2 Ã— (-3)) + (2- – 1)3]

= 10[-2 + 57]

= 10 Ã— 55

= 550

**20. The sum of first 14 terms of an A.P. is 1505 and its first term is 10. Find its 25 ^{th} term.**

**Solution:-**

From the question it is given that,

First term a = 10

The sum of first 14 terms of an A.P. = 1505

25^{th} term = ?

We know that, S_{n} = (n/2) [2a + (n – 1)d]

S_{14} = (n/2) [2a + (n – 1)d]

1505 = (14/2) [(2 Ã— 10) + (14 – 1)d]

1505 = 7[20 + 13d]

1505/7 = 20 + 13d

215 = 20 + 13d

13d = 215 â€“ 20

13d = 195

d = 195/13

d = 15

Then, a_{n }= a + (n – 1)d

a_{25} = 10 + (25 – 1)(15)

= 10 + (24)15

= 10 + 360

= 370

**21. Find the geometric progression whose 4 ^{th} term is 54 and 7^{th} term is 1458.**

**Solution:-**

From the question it is given that,

The geometric progression whose 4^{th} term a_{4} = 54

The geometric progression whose 7^{th} term a_{7} = 1458

We know that, a_{n} = ar^{n â€“ 1}

a_{4} = ar^{4 â€“ 1}

a_{4} = ar^{3} = 54

a_{7}^{ }= ar^{6} = 1458

By dividing both we get,

ar^{6}/ar^{3} = 1458/54

r^{6 â€“ 3} = 27

r^{3} = 3^{3}

r^{ }= 3

To find out a, consider ar^{3} = 54

a(3)^{3} = 54

a = 54/27

a = 2

Therefore, a = 2, r = 3

So, G.P. is 2, 6, 18, 54,â€¦

**22. The fourth term of a G.P. is the square of its second term and the first term is â€“ 3. Find its 7 ^{th} term.**

**Solution:-**

From the question it is given that,

The fourth term of a G.P. is the square of its second term = a_{4} = (a_{2})^{2}

The first term a_{1} = – 3

We know that, a_{n} = ar^{n â€“ 1}

a_{4} = ar^{4 â€“ 1}

a_{4} = ar^{3}

a_{2} = ar

Now, ar^{3} = (ar)^{2}

ar^{3} = a^{2}r^{2}

r^{3}/r^{2} = a^{2}/a

r^{3 â€“ 2} = a^{2 â€“ 1} â€¦ [from a^{m}/a^{n}Â = a^{m – n}]

r = a

a_{1 }= -3

a_{7} = ar^{7 â€“ 1}

a_{7} = ar^{6}

= -3 Ã— (-3)^{6}

= -3 Ã— 729

= -2187

Therefore, the 7^{th} term a_{7} = -2187

**23. If the 4 ^{th}, 10^{th} and 16^{th} terms of a G.P. are x, y and z respectively, prove that x, y and z are in G.P.**

From the question it is given that,

a_{4} = x

a_{10} = y

a_{16} = z

Now, we have to show that x, y and z are in G.P.

We know that,

a_{n} = ar^{n – 1}

a_{4} = ar^{4 â€“ 1}

a_{4} = ar^{3} = x

a_{10} = ar^{9} = y

a_{16} = ar^{15} = z

x, y, z are in G.P.

If y^{2} = xy

Substitute the value of x and y,

y^{2} = (ar^{9})^{2}

y^{2} = a^{2}r^{18}

Then, xz = ar^{3} Ã— ar^{15}

= a^{1 + 1} r^{3 + 15} â€¦ [from a^{m} Ã— a^{n} = a^{m + n}]

= a^{2}r^{18}

So, y^{2} = xy

Therefore, it is proved that x, y, z are in G.P.

**24. How many terms of the G.P. 3, 3/2, Â¾ are needed to give the sum 3069/512?**

**Solution:-**

From the question it is given that,

Sum of the terms S_{n} = 3069/512

First term a = 3

Common ratio r = (3/2)/3

= (3/2) Ã— (1/3)

= Â½

We know that, S_{n} = a(1 â€“ r^{n})/(1 – r)

(3069/512) = 3[1 â€“ (Â½)^{n}]/ (1 â€“ Â½)

(3069/512) = (2 Ã— 3) [1 â€“ (Â½)^{n}]

1 â€“ (Â½)^{n} = 3069/(512 Ã— 6)

1 â€“ (Â½)^{n} = 1023/1024

(Â½)^{n} = 1 â€“ (1023/1024)

(Â½)^{n} = (1024 – 1023)/1024

(Â½)^{n} = 1/1024

(Â½)^{n} = (Â½)^{10}

By comparing both LHS and RHS,

n = 10

Therefore, there are 10 terms are in the G.P.