charge on concentric shells

Potential of large spherical shell is,
V(shell) = kQ/R + kq/R
where k = 1/(4.pi.e), e is the permitivity of free space and pi = 3.14159.....
Potential of small sphere is
V(sphere) = kQ/R + kq/r
hence, potential difference between the two conductors
= V(sphere) - V(shell)
= (kQ/R + kq/r) - (kQ/R + kq/R)
= kQ/R + kq/r - kQ/R - kq/R
= kq(1/r - 1/R)
= q/(4.pi.e)(1/r - 1/R)