（1）(x+y)·x^2-y^2分之x^2 +x-y分之y^2 -（2x^2y-2xy^2)÷(x^2-2xy+y^2）=(x+y)*x²/(x+y)(x-y)+y²/(x-y)-2xy(x-y)/(x-y)²=x²/(x-y)+y²/(x-y)-2xy/(x-y)=(x²+y²-2xy)/(x-y)=(x-y)²/(x-y)=x-y
