Find the sum of the first twenty term of the g.p whose third term is 4.5 and 5th term is 7.625

Answer 1

r^2 = 7 5/8 ÷ 4 1/2 = 61/36
r = ±√61/6
So, a = 162/61
And, as usual,
S20 = a(r^20 - 1)/(r-1)