Example 1.31
Find the sum of the infinite geometric series:
e−1 + e−2 + e−3 + e−4 + · · · (1.186)
The first term and the common ratio are a1 = e−1 and r = e−1 < 1. The
sum is therefore given by
−1
e 1
S = −1 = . (1.187)
1 − e e − 1
Example 1.32
Find the sum of the infinite geometric series:
2 3
n n n
x + x + x + x + · · · , 0 < n < 4. (1.188)
4 16 64
It is obvious that a1 = x and r = n < 1. Consequently, the sum of this
4
infinite series is given by
x 4
S = n = x, 0 < n < 4. (1.189)
1 − 4 − n
4