If X has a discrete distribution and Y has a continuous distribution, we can derive the marginal p.f. of X and the marginal p.d.f. of Y from the joint p.f./p.d.f. in the same ways that we derived a marginal p.f. or a marginal p.d.f. from a joint p.f. or p.d.f. Let f be the joint p.f./p.d.f. of X and Y. Then the marginal p.f. of X is