Its better that you show us your attempt rather than us do it for you. First try using the substitution formula and calculating the Jacobian and post it in this thread.
Hey there I need some help with this problem
Let be the region 0 <(or equal to) y <(or equal to) x
0 <(or equal to) x <(or equal to) 1
double integral over D of (x+y) dxdy
by making the change of variables x=u+v, and y=u-v
Check your answers by evaluating the iterated integral directly by using an iterated integral.
Please explain each step you did, so I can understand how to do this problem thanks.
As a start, x + y = u + v + (u - v) = 2u. Now find the limits in terms of u and v for x = 0 to 1 and y = 0 to x.
Also consider when you make a change of the order of integration (as this may explain why they want to use the new parameterization in terms of u and v).