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
其中x=8,y等于3分之1
(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)²x²]
=-(x-y)²x²y/[(x-y)²x²]
=-y
=-1/3
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