Intermediate Algebra (6th Edition)

$S_{8} = 85$
Given geometric sequence $-1,2,-4,...$ $a_{1} = -1$ Common ratio $r = \frac{a_{n}}{a_{n-1}}$ $r= \frac{a_{2}}{a_{1}} = \frac{2}{-1} = -2$ Sum of first eight terms is $S_{8} = \frac{-1(1-(-2)^{8})}{1-(-2)}$ [Using $S_{n} =\frac{ a_{1}(1-r^{n})}{1-r}$] $S_{8} = \frac{-1(1-256)}{1+2}$ $S_{8} = \frac{-1(-255)}{3}$ $S_{8} = \frac{255}{3}$ $S_{8} = 85$