：(x^2+2xy+y^2)/(x^2y+xy^2)-(x^2-2xy+y^2)/(x^2y-xy^2)
=(x+y)^2/[xy(x+y)]-(x-y)^2/[xy(x-y)]
=(x+y)/xy-(x-y)/xy
=(x+y-x+y)/xy
=2y/xy
=2/x