Answer
$-21486$
Work Step by Step
The sum of a geometric sequence until $n$ can be obtained by the formula $S_n=a_1(\frac{1-r^n}{1-r})$ where $a_1=-2$ is the first term and $r=-2$ is the common ratio. Hence here the sum: $S_{15}=-2(\frac{1-(-2)^{15}}{1-(-2)})=-21486$
