Question

Find the sum of the geometric sequence –3, 15, –75, 375, … when there are 8 terms and select the correct answer below.

Answer 1

–195,312
195,312<-----my answer
292,968
–292,968

Answer 2

good choice.

Answer 3

a=3
\[r=\frac{ 15 }{ 3 }=-5\]
\[S _{n}=\frac{ a \left( r^n-1 \right) }{ r-1 }\]
\[S _{8}=\frac{ -3\left\{ \left( -5 \right)^8-1 \right\} }{ -5-1 }\]
?