（x^2+y^2)/(x-y)-2xy/(x-y)=(x^2+y^2-2xy)/(x-y)=(x-y)^2/(x-y)=x-y
（x²+y²)/(x-y)-2xy/(x-y)=(x²+y²-2xy)/(x-y)=(x-y)²/(x-y)=x-y
