Question

What is the sum of the geometric sequence 8, -16, 32 . if there are 15 terms?

Answer 1

a=8
r=-16/8=-2
\[S _{n}=a \frac{ r^n-1 }{ r-1 }\]
put n=15 and get the result.

Answer 2

I dont understand how to do that

Answer 3

\[S _{15}=8\frac{ \left( -2 \right)^{15}-1 }{ -2-1 }=8\frac{ -32768-1 }{ -3 }=8\frac{ 32769 }{ 3}=8*10923=87384\]

Answer 4

Another question: What is the sum of the geometric sequence 4, 12, 36 . if there are 9 terms?

Answer 5

here a=4, r=12/4=3
n=9
use the above formula