During the derivation of the sum of the first n terms of a GP with first term a and common ratio r(r=1), one arrives at
rSn−Sn=arn−a.
Which of the following is the correctly simplified expression for Sn obtained from this equation?
a) ar−11−rn
b) r−1rn−1
c) ar−1rn−1
d) ar−1rn+1−1