If x=1+a+a2+a3+... and y=1+b+b2+b3+...., then show that 1+ab+a2b2+a3b3+...=xy/x+y−1

How to solve?
Q.If x=1+a+a2+a3+... and y=1+b+b2+b3+...., then show that 1+ab+a2b2+a3b3+...=xy/x+y−1
CH- (Geometric Progressions) | IMPORTANT QUESTIONS