先化简再求值(xy-x的2次方)除以xy分之（x的2次方-2xy+y的2次方）成x的2次方分之x-y

(xy-x的2次方)除以xy分之（x的2次方-2xy+y的2次方）成x的2次方分之(x-y)?=(xy-x的2次方)/[(x的2次方-2xy+y的2次方)/(xy)]*[(x-y)/x²]=-x(x-y)/[(x-y)²/(xy)]*[(x-y)/x²]=-x(x-y)*xy*(x-y)/[(x-y)...