2 2 x - y ------- 2 2 x + y
(x^2 - y^2)/(x^2 + y^2)
(x + y)*(x - y) --------------- 2 2 x + y
(x + y)*(x - y)/(x^2 + y^2)
2 2*y 1 - ------- 2 2 x + y
1 - 2*y^2/(x^2 + y^2)