Question

1. Find Sn of a geometric sequence if:

n = 6, a1 = 16 and a3 = 4

Answer 1

Explanation:

Sn = a1 * (r^n - 1) / (r-1)

a3 = a1r^2 = 4, and

a1 = 16 , so:

r^2 = 4/16 = 0.25

r = 0.5

S6 = 16 * (0.5^6 - 1) / (-0.5)

= 16 * 1.96875

= 31.5.