Answer
geometric,
$r=4$
$S_n=-\frac{2(4^n-1)}{3}$
Work Step by Step
1. Based on the given sequence, we can determine it is geometric,
2. We can find the common ratio as $r=\frac{s_2}{s_1}=(\frac{-1/2}{-1/2})\frac{4^2}{4^1}=4$
3. We can find the sum of the first n terms as $S_n=\frac{s_1(r^n-1)}{r-1}=\frac{(-2)(4^n-1)}{4-1}=-\frac{2(4^n-1)}{3}$
