I'm trying to do an exercise of joint probability distributions in which I ask to show that the pdf is a valid pdf
$ f(x,y) =\left\{\begin{array}{cl} \frac{3(x^2+y^2)}{16} & \text{if }0
to demonstrate that it is a valid pdf I have to integrate all $\mathbb R$, however as the function is non-zero only on the interval $0