Question

What is the sum of the geometric sequence -3 18 -108.... If there are 8 terms?

Answer 1

`S_8=719835`

Explanation:

To find the sum of the first `S_n` terms of a geometric sequence, use the formula `S_n=(a(1-r^n))/(1-r)` where `n` is the number of terms `a` is the first term and `r` is the common ratio.

`a=-3, r=-6 and n=8`

`S_8=((-3)(1-(-6)^8))/(1-(-6))`

`S_8=((-3)(1-1679616))/(7)`

`S_8=((-3)(-1679615))/(7)`

`S_8=719835`