You visited us 1 times! Enjoying our articles? Unlock Full Access!

Sum of 'n' Terms When 'a' and 'l' Is Given

Question 10 i...

Question

Question 10 (i)
Show that $a_{1}, a_{2} \dots \dots, a_{n}, \dots \dots$ form an AP where $a_{n}$ is defined as below.
(i) $a_{n} = 3 + 4 n$
Also find the sum of the first 15 terms in each case.

Open in App

Solution

$G i v e n, a_{n} = 3 + 4 n a_{1} = 3 + 4 (1) = 7 a_{2} = 3 + 4 (2) = 3 + 8 = 11 a_{3} = 3 + 4 (3) = 3 + 12 = 15 a_{4} = 3 + 4 (4) = 3 + 16 = 19 I t c a n b e o b s e r v e d t h a t; a_{2} - a_{1} = 11 - 7 = 4 a_{3} - a_{2} = 15 - 11 = 4 a_{4} - a_{3} = 19 - 15 = 4 i . e ., a_{k + 1} - a_{k} i s s a m e e v e r y t i m e . T h e r e f o r e, t h i s i s a n A P w i t h c o m m o n d i f f e r e n c e a s 4 a n d f i r s t t e r m a s 7. S_{n} = \frac{n}{2} [2 a + (n - 1) d] S_{15} = \frac{15}{2} [2 (7) + (15 - 1) \times 4] = \frac{15}{2} [(14) + 56] = \frac{15}{2} (70) = 15 \times 35 = 525$

Suggest Corrections

Similar questions

a_{1}, a_{2} \dots \dots, a_{n}, \dots \dots

a_{n}

a_{n} = 3 + 4 n

a_{1}, a_{2} \dots \dots, a_{n}, \dots \dots

a_{n}

a_{n} = 3 + 4 n

a_{1}, a_{2} \dots \dots, a_{n}, \dots \dots

a_{n}

a_{n} = 9 - 5 n

Show that a₁, a₂… , a_n , … form an AP where a_n is defined as below

(i) a_n = 3 + 4n

(ii) a_n = 9 − 5n

Also find the sum of the first 15 terms in each case.

a_{1}, a_{2}, . . ., a_{n}

a_{n}

a_{n} = 3 + 4 n

a_{n} = 9 - 5 n

15

Join BYJU'S Learning Program

Explore more

Join BYJU'S Learning Program