If s is the sum of an infinite G.P, the first term a then the common ratio r given by

(1) (a-s)/s

(2) (s-a)/s

(3) a/(1-s)

(4) (s-a)/a

Solution:

The sum of an infinite GP, s = a/(1-r)

s(1-r) = a

s-sr = a

(s-a) = sr

r = (s-a)/s

Hence option (2) is the answer.