Question

Find S8.
3+-6+12-...

Answer 1

\[a,ar,ar^2 ...\]
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

\[\frac{3(1+128)}{3}=129\]

Answer 2

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