I have a query related to a transformation of variables question; the images of which I have attached here.
I have solved the question almost, but there seems to be a problem with the distribution that I have derived in the last line of the solution.
I would greatly appreciate any kind of help. Thanks in advance.
N.B.: In the question, $U$ should follow $N(\mu = 0, \sigma^2 = \frac{1}{4} = 0.25)$.


Can you please explain as to why the transformation (x,y) -> (u,v) not one-to-one ? Also, we are mapping from X and Y. Why have you considered here -X and -Y ?
– Dwaipayan Gupta Mar 23 '18 at 19:25