# Joint pdf question and mgf

by silkdigital
Tags: joint
 P: 4 Hi guys, I'm really stuck on the following questions, not sure as to how to approach it: Let X and Y be random variables for which the joint pdf is as follows: f(x,y) = 2(x+y) for 0 <= x <= y <= 1 and 0 otherwise. Find the pdf of Z = X + Y And also: Suppose that X is a random variable for which the mgf is as follows: /u(t) = e^(t^2 + 3t) for minus infinity < t < infinity Find the mean and variance for X. I know that the answers are 3 and 2 respectively, but was unsure how they got to the answer, do I need to integrate by parts? Any help would be appreciated! Thanks guys :)