given a complicated p.d.f of x,y. How do you find the p.d.f of X?
i.e.
kx^(2)e^(-ax)e^(-xy) x>0 & y>0
The marginal distribution of one of the random variables is obtained by integrating the joint pdf with respect to the other variable
$\displaystyle \displaystyle f_{X}(x) = \int_Y f(x,y)\;dy\;=\; .....$