Hey lxw291.

How do you express r in terms of your x's and y's?

You will have r as a function of (x,y) and theta as a function of (x,y): The integral substitution will involve a Jacobian term and your substitutions will just substitute these functions.

Multiple integral - Wikipedia, the free encyclopedia

Just like you do a substitution for a,b -> x(a), x(b), you do the same thing to take (x,y) to r(x,y) theta(x,y) even if your limits are variable ones and not constant.