Question

The series has a sum to infinity of 9 and the sum of the first two terms is 5. Find the first four terms of the series

Answer 1

a+ar = 5

a/(1-r) = 9
so (1+r)(1-r) = 5/9
r = ±2/3
Now find a, and then you want
S4 = a(1-r^4)/(1-r)