Question

The sum of the first and second terms of an geometry series is 9.the sum to infinit is 12.calculate the possible values of r.

Answer 1

a + ar = 9

a(1+r) = 9
a = 9/(1+r)

a/(1-r) = 12
a = 12(1-r)

12(1-r) = 9/(1+r)
1 - r^2 = 3/4
4 - r^2 = 3
-r^2 = -1
r = ± 1