Evaluate $\iint_{0Let $$f(x,y)=\begin{cases}xy &\text{ if } 0<x<y<1, \\ 0 &\text { otherwise. }\end{cases}$$  Evaluate the  integral $\displaystyle \iint f(x,y)\,dx\,dy$.
I'm having trouble with the limits on integration. 

HINT
your inequality $0<x<y<1$ indicates you can integrate $\int_0^y fdx$ first. What are the correct limits on the outer $dy$ integral then.
