Question

Find the sum of the geometric sequence -3,15,-75,375,... when there are 9 terms

Answer 1

-976,563.

Explanation:

The common ratio is 15/-3 = -75/15 = 375/-75 = -5.

Sum of n terms = a1. (r^n - 1) / (r - 1)

so S9 = -3 * ((-5)^9 - 1)) / (-5 - 1)

= -976,563.