## anonymous 5 years ago Find S8. 3+-6+12-...

$a,ar,ar^2 ...$ for our case a=3 r=-2 sum of first n terms is $\frac{a\left(1-r^{(n+1)}\right)}{1-r}$ so by substituing, we get $\frac{3(1+128)}{3}=129$

2. dumbcow

incorrect $=\frac{a(1-r ^{n}}{1-r}$ $=\frac{3(1-(-2)^{8})}{3}$ = 1-256 =-255