The sum of the first n terms of a geometric sequence is given by
a1(1 − rn)
Sn = , r = 1. (1.181)
1 − r
An infinite geometric series converges if and only if |r| < 1. Otherwise it
diverges. The sum of infinite geometric series, for |r| < 1, is given by
a1
lim Sn = , (1.182)
n→∞ 1 − r
obtained from (1.181) by noting that
lim rn = 0, |r| < 1. (1.183)
n→∞
Some of the methods that will be used in this text may give the solutions
in an infinite series. Some of the obtained series include infinite geometric
series. For this reason we will study examples of infinite geometric series.
Example 1.30
Find the sum of the infinite geometric series
2 4 8
1 + + + + · · · (1.184)
3 9 27
The first value of the sequence and the common ratio are given by a1 = 1
and r = 2 respectively. The sum is therefore given by
3
1
S = 2 = 3. (1.185)
1 −
3