## Precalculus (6th Edition) Blitzer

$\frac{27}{2}=13.5$
The sum of a infinite geometric sequence is given as: $S=\dfrac{a_1}{1-r}$ Here, $a_1=9$ and Common Ratio: $r=\dfrac{1}{3}$ $S=\dfrac{a_1}{1-r}$ Now, $S=\dfrac{9}{1-\dfrac{1}{3}}=\dfrac{27}{3-1}=\frac{27}{2}=13.5$