Question

Find the sum of the geometric sequence –3, 15, –75, 375, … when there are 8 terms

Answer 1

First, we need to solve for the common ratio from the data given by using the equation.

a(n) = a(1) r^(n-1)
15 = -3 r^(2-1)
-5 = r
r = -5

Then, we can find the sum by the expression:

S(n) = a(1) ( 1 - r^n) / 1-r
S(8) = -3 (1 + 5^8) / 1+5
S(8) = -195313