求值:[x/(x-y)-1]/[y^2/(x+y)]*[x-x^2/(x+y)],其中，x=-2,y=-1
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原式=[(x-x+y)/(x-y)]/[y^2/(x+y)]*[(x^2+xy-x^2)/(x+y)]
=[y/(x-y)]*[(x+y)y^2]*[xy/(x+y)]
=x/(x-y)
=[y/(x-y)]*[(x+y)y^2]*[xy/(x+y)]
=x/(x-y)
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