need help with double integral

hey guy, I'm having a problem with double integral integration. Can you guys tell me where I went wrong?

The equation is

let

However, I seem like this answer is wrong, and the correct answer should be

Can anyone point out what I did wrong here?

Best Regards

Junks

Re: need help with double integral

$(x+y)^2 = x^2 + y^2$

Really? Try it again.

Re: need help with double integral

ahh, I see, I'll let U=x^2 + y^2

.................................................. ..........................................

So I let

let and

and

using this rule

the answer turn out to be -0.38, but the correct answer seem to be pi/12 or 0.26. What did I do wrong in here?

Re: need help with double integral