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Grade: 11


Please explain the concept of finding the sum of an infinite Geometric Progression.

5 years ago

Answers : (1)

Latika Leekha
askIITians Faculty
165 Points
							Hello student,
First of all a geometric progression or a geometric series is a series of terms in which the ratio of any two adjacent terms is constant.
Eg. 2,,4 8, ….. is a geometric progression where the common ratio i.e. r = 2.
Now we discuss how to find the sum of infinite Geometric Progression:
Firstly, the sum of n terms of a G.P. is given by
S = a(1-rn)/(1-r), where r ≠ 1.
Now, if |r| < 1 and n → ∞ then rn → 0 and in this case geometric series will be summable upto infinity and its sum is given by S= a/(1-r).
I’ll consider one example here based on this concept:

Eg: The sum of an infinite number of terms of a G.P. is 15 and the sum of their squaresis 45. Find the series.
Sol: We have a + ar + ar2 + ar3 + ....... = 15
so, a/(1-r) = 15.
a2 + (ar)2 + (ar2)2 +.... = 45

242-255_forum 4th june.png
(1-r)/(1+r) = 1/5
Solving this we get 5-5r = 1+r.
So r = 2/3.
so, a = 15(1-r) = 15(1-2/3) = 5.
so the series is 5, 10/3, 20/9, ….. .
5 years ago
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