Chapter 11 - Review - Page 668: 56

$S_{\infty}=\dfrac{81}{5}$

Using $S_{\infty}=\dfrac{a_1}{1-r}$, the sum of the infinite geometric sequence $ 18,-2,\dfrac{2}{9},... $ is \begin{array}{l} S_{\infty}=\dfrac{18}{1-\left(-\dfrac{1}{9}\right)}\\\\ S_{\infty}=\dfrac{18}{\dfrac{10}{9}}\\\\ S_{\infty}=\dfrac{81}{5} .\end{array}