Question

What is the sum of the first nine terms of the geometric series where a1 = 7 and r = −3?

Answer 1

Sum of terms of a geometric series
Sn = a1 (r^n-1)/(r-1)
a1 = 1st term = 7
n = number of terms = 9
r = common ratio  = -3

Sn = 7 ((-3)^8-1)/(-3-1) = -11480
Hence the sum is -11480.