2xy/(x+y)^2=2xy(x-y)/[(x+y)^2(x-y)]=(2x^2y-2xy^2)/[(x+y)^2(x-y)]
x/(x^2-y^2)=x(x+y)/[(x+y)(x-y)*(x+y)=(x^2+xy)/[(x+y)^2(x-y)]
最简公分母是(x+y)^2(x-y)