f1 (x, y) = x/(x2 + y2), f2(x, y) = y/(x2 + y2), °Jf (x, y) =      1
---2---2-2
(x  + y )(y2 - x2   - 2xy )
            2    2
  - 2xy   x  - y ,   det(Jf (x, y)) = -----1-----
(x2 + y2)2.