In the proof of Theorem 5.2 we determined the quantity E[X(X – 1)], called the second factorial moment. In general, the rth factorial moment of X is given by
µ'(r) = E[ X(X – 1)(X – 2) · . . . · (X – r + 1)]
Express µ'2, µ'3, and µ'4 in terms of factorial moments.