Since a, b, c are in an arithmetic progression, we have a = b - d, c = b + d. Since x, y, z are in an geometric progression, we have x = y*r^(-1), z = y*r. Now, x^b*y^c*z^a =. . = . = . . Next, x^c*y^a*z^b = . . = . = . . Right sides are the same. So are left sides. Proved.