Find the PDF of Z = X - Y. There seems to be some confusion in the notation for problem 5.49 (and others). When I say that f_x(v)=f_Y(v), I'm just trying to say that the PDFs of X and Y have the same shape. I am not implying that they are somehow functions of some other variable V. Little v was just the independent variable. You can give it any name you want (like x or y). So: f_x(x)-bexp(-bx)u(x). f_Yly)-bexp(-by)u(y). Then f_{XY](x,y)-b^2 exp(-b(x+y))u(x)u(y). Let X and Y be independent and both exponentially distributed with fx(v) =fy(v) = bebvu(v).