I am wondering if someone could help me with the following? I am supposed to show that ∫∫f(x+y)dA evaluated from the triangular region with the vertices (0,0), (1,0) and (0,1) is equal to ∫∫uf(u)du. This triangular region has the equations, x = 0, x = 1, and y = -x + 1. If I set x+y = u, then I know that u must satisfy the equations u = 1, u = 0. But, I’m not sure what to do after that? If someone could give me some hints, that be great. Thanks!!