Does anyone know of any sources that explain change of variables for double integrals. Actually, I get the change of variables thing, but a few of our problems don't give us the transforms. I don't understand how to create these myself. Here is an example: Math Problem So far, I found all the x,y coordinates of the joints because I know these joints or cross sections will exist after we change variables. However, I don't know where to go from there. Can I essentially make any transform I like?