Question

What is the sum of the geometric series in which a1 = −5, r = 3, and an = −405?

A hint on how to solve is attached!
r ≠ 1, where a1 is the first term and r is the common ratio.

Answer 1

an = a1 * r^(n-1)

-405 = -5*3^(n-1)

3^(n-1) = 81

3^4 = 81 so n-1 = 3 and n = 4

Sn = S4 = -5 (1 - 3^4) / ( 1 - 3) = 400 / -2 = -200 Answer