(Practice with joint, marginal and conditional densities) This is a toy problem designed to give you practice in working with a number of the concepts we've examined; in a course like this, every now and then you have to stop looking at real-world problems and just work on technique (it's similar to classical musicians needing to practice scales in addition to actual pieces of symphonic or chamber music) Suppose that the continuous random vector X - (Xi, X2) has PDF given by 412 for 0