Chapter 9, Problem 30E

Obtain equation (9−15) from (9−14). Make use of the following sums, correct when $\left|x\right|<1$ :

${\displaystyle \underset{n=0}{\overset{\infty }{\sum }}{x}^n=\frac{1}{1-x}}$

${\displaystyle \underset{n=0}{\overset{\infty }{\sum }}n{x}^n=\frac{x}{{\left(1-x\right)}^2}}$