1. The problem statement, all variables and given/known data Calculate the marginal density of fx (x). Fx,y (x,y) = double integral of 4x.e^(x+y) when 0<x<y, 0 otherwise. 2. Relevant equations 3. The attempt at a solution i set up the equation as fx (x) = integral from x up to infinity of 4x.e^(x+y) dy i get 4x [e^(x+y)] from x to infinity, thats when the problem arises as whatever i do there's always infinity in the question. I'm told the answer is 4x.e^(-2x) any help would be really appreciated. thanks.