First sequence is a,ar,ar2,ar3,...,arn−1
Second sequence is A,AR,AR2,AR3,...,ARn−1
Let P= product of corresponding terms of the first and second sequence.
=aA,arAR,ar2AR2,ar3AR3,...,arn−1ARn−1
Now,arARaA=rR and ar2AR2arAR=rR
Thus, the common ratio is the same.
thus, it is a G.P.
∴ Common ratio=rR