A metal sphere with radius r_a is supported on an insulating stand at the center of a hollow, metal spherical shell with radius r_b. There is charge + q on the inner sphere and charge - q on the outer spherical shell. Take the potential V to be zero when the distance r from the center of the spheres is infinite.
Calculate the potential V(r) for r > r_{rm b}. (Hint: The net potential is the sum of the potentials due to the individual spheres.)