Find the sum of the terms of the series
$1, \frac{4}{5}, \frac{16}{25}, \frac{64}{125}, \ldots \ldots \ldots \infty$
25
135
5
125
It can be noticed that the given series is a G.P. with infinite terms. The common ratio is (4/5) and this is < 1.
The sum to infinity of a series with first term a and the common ratio
$r=[a /(1-r)]$
As $a=1$ and $r=4 / 5$
the sum to infinity $=\frac{1}{1-(4 / 5)}=5$