Thanks for your information, tiny-tim.
I have a bit more thoughts bout the problem.
P(X+Y<b, X<a)
Let U=X-a, V=Y-b+a, then, we have
P(U+V<0, U<0), In this form, the integral area is a sector, we can use polar coordinates to solve it., Such as,
P(U+V<0, U<0)=\int_{\theta}^{3\pi...