先化简.再求值 x分子x减y除与括号x减x分之2xy减y的平方 其中x等于2 y等于负1

x分之(x-y) ÷ [x- x分之(2xy-y²)]
=x分之(x-y)÷[x分之(x²-2xy+y²)]
=x分之(x-y)* [(x-y)²分之x]
=(x-y)分之1
=（2+1）分之1
=3分之1