If the sum of the n terms of G.P. is S product is P and sum of their inverse is R, than P2 is equal to
S=a(1−rn)1−rP=anrn(n−1)2R=1a+1ar+1ar2+⋯⋯+1arn−1=1a(1−1rn)1−1r=1arn−1(rn−1r−1)p2=a2nrn(n−1)(SR)=a2nrn(n−1)