Answer
$S_9=855$
Work Step by Step
The given sequence $
5,-10,20,...
$ is a geometric sequence with $a_1=5$ and $r=-2$. Using $S_n=\dfrac{a_1(1-r^n)}{1-r}$, then,
\begin{array}{l}
S_9=\dfrac{5(1-(-2)^9)}{1-(-2)}\\\\
S_9=\dfrac{5(513)}{3}\\\\
S_9=855
.\end{array}
