Suppose that X, Y, Z are discrete random variables taking values in the natural numbers with joint p.m.f. fx,y.z(i, j, k) and joint c.d.f. Fx,y,z(i, j, k) (written F(i, j, k) for short). Show that fx,y.z(i, j, k) is equal to F(i, j, k) - F(i - 1, j, k) - F(i, j - 1, k) - F(i, j, k - 1) + F(i, j - 1, k - 1) + F(i - 1, j, k - 1) + F(i - 1, j - 1, k) - F(i - 1, j - 1, k - 1)