If a, b, c are pth, qth, and rth terms of a G.P., then (cb)p(ba)r(ac)q is equal to
1
a = ARp−1, b = ARq−1, c = ARr−1
∴(cb)p(ba)r(ac)q = (ARp−1ARq−1)p(ARq−1ARp−1)r(ARp−1ARr−1)q
= R(r−q)p+(q−p)r+(p−r)q = R0 = 1