Geometric Progression

In Maths, Geometric Progression (GP) is a type of sequence where each succeeding term is produced by multiplying each preceding term by a fixed number, which is called a common ratio. This progression is also known as a geometric sequence of numbers that follow a pattern. Also, learn arithmetic progression here.

The common ratio multiplied here to each term to get a next term is a non-zero number. An example of GP is 2, 4, 8, 16, 32, 64, …, where the common ratio is 2.

Definition

As discussed in the introduction, a geometric progression or a geometric sequence is the one, in which each term is varied by another by a common ratio. The next term of the sequence is produced when we multiply a constant (which is non-zero) to the preceding term. It is represented by:

a, ar, ar2, ar3, ar4, and so on.

Where a is the first term and r is the common ratio.

Note: It is to be noted that when we divide any succeeding term from its preceding term, then we get the value equal to common ratio.

Suppose we divide 3rd term by 2nd term we get:

ar2/ar = r

In the same way:

ar3/ar2 = r

ar4/ar3 = r

General Form of GP

The general form of Geometric Progression is:

a, ar, ar2, ar3, ar4,…,an

Where,

a = First term

r = common ratio

an = nth term

General Term or Nth Term of GP

Let a be the first term and r be the common ratio for a G.P.

Then the second term, a2 = a × r = ar

Third term, a3 = a2 × r = ar × r = ar2

Similarly, nth term, an = arn-1

Therefore, the formula to find the nth term of GP is:

an = arn-1

Note: nth term is the last term of GP.

Common Ratio

Consider the sequence a, ar, ar2, ar3,……

First term = a

Second term = ar

Third term = ar2

Similarly nth term, tn = arn-1

Thus, Common ratio = (Any term) / (Preceding term)

= tn / tn-1

= (arn – 1 ) /(arn – 2)

= r

Thus, the general term of a G.P is given by arn-1 and the general form of a G.P is a + ar + ar2 + …..

For Example: r = t2 / t1 = ar / a = r

Sum of N term of GP

Suppose a, ar, ar2, ar3,……arn-1 is the given Geometric Progression.

Then the sum of n terms of GP is given by:

Sn = a+ar+ar2+ ar3+…+arn-1

The formula to find sum of n terms of GP is:

Sn = a[(rn-1)/(r-1)] if r ≠ 1

Where

a is the first term

r is the common ratio

n is the number of terms

Also, if the common ratio is equal to 1,then the sum of the GP is given by:

Sn = na if r=1

Infinite Geometric Progression

The terms of a finite G.P. can be written as a, ar, ar2, ar3,……arn-1

a, ar, ar2, ar3,……arn-1 is called finite geometric series.

The sum of finite Geometric series is given by:

Sn = a[(rn-1)/(r-1)] if r ≠ 1

Terms of an infinite G.P. can be written as a, ar, ar2, ar3, ……arn-1,…….

a, ar, ar2, ar3, ……arn-1,……. is calledinfinite geometric series.

The sum of infinite geometric series is given by:

\(\sum_{k=0}^{\infty}\left(a r^{k}\right)=a\left(\frac{1}{1-r}\right)\)

Solved Examples of Geometric Progression

Question 1: If the first term is 10 and the common ratio of a GP is 3. Then write the first five terms of GP.

Solution: Given,

First term, a = 10

Common ratio, r = 3

We know the general form of GP for first five terms is given by:

a, ar, ar2, ar3, ar4

a = 10

ar = 10 x 3 = 30

ar2 = 10 x 32 = 10 x 9 = 90

ar3 = 10 x 33 = 270

ar4 = 10 x 34 = 810

Therefore, the first five terms of GP with 10 as first term and 3 as common ratio is:

10, 30, 90, 270 and 810

Question 2: Find the sum of GP: 10, 30, 90, 270 and 810, using formula.

Solution: Given GP is 10, 30, 90, 270 and 810

First term, a = 10

Common ratio, r = 30/10 = 3

Number of terms, n = 5

Sum of GP is given by;

Sn = a[(rn-1)/(r-1)]

S5 = 10[(35-1)/(3-1)]

= 10[(243-1)/2]

= 10[242/2]

=10 x 121

=1210

Check: 10+30+90+270+810 = 1210

Question 3: If 2,4,8,…., is the GP, then find its 10th term.

Solution: The nth term of GP is given by:

an = arn-1

Here, a = 2 and r = 4/2 = 2

Therefore,

a10 = 2 x 22-1

= 2 x 2

= 4

Frequently Asked Questions on Geometric Progression

What is a Geometric Progression?

Geometric Progression (GP) is a type of sequence where each succeeding term is produced by multiplying each preceding term by a fixed number, which is called a common ratio.

Give an example of Geometric Progression?

The example of GP is: 3, 6, 12, 24, 48, 96,…

What is the general form of GP?

The general form of Geometric Progression is given by a, ar, ar2, ar3, ar4,…,an

a = First term

r = common ratio

an = nth term

What is the common ratio in GP?

The common multiple between each successive term and preceding term in a GP is the common ratio. It is a constant value which is multiplied by each term to get the next term in Geometric series. If a is the first term and ar is the next term, then the common ratio is equal to:

ar/a = r

What is not a geometric progression?

If the common ratio between each term of a geometric progression is not equal then it is not a GP.

What is the sum of a geometric series?

If a, ar, ar2, ar3,……arn-1 is the given Geometric Progression, then the formula to find sum of GP is:

Sn = a+ar+ar2+ ar3+…+arn-1

Or

Sn= a[(rn-1)/(r-1)] where r ≠ 1

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