Let $X$ and $Y$ be random variables with joint density function $$f(x,y) = \begin{cases} 2 e^{-2 x}/x, & 0\le x<\infty, 0\le y\le x\\ 0, & \mbox{otherwise}.\end{cases}$$ Compute $\mbox{Cov}(X,Y)$.
$\mbox{Cov}(X,Y) =$