（x-y)/(2x+2y)
=(x-y)/2(x+y)
=(x-y)(x+y)/2(x+y)^2
xy/(x+y)^2
=2xy/2(x+y)^2
x - y/2x + 2y
=(x^2-y+4xy)/2x
=(x^2-y+4xy)(x+y)^2/2x(x+y)^2
xy/(x+y)^2
=2x^2y/2x(x+y)^2
不知你的分式是哪種情況.