当x=0.3,y=0.7时，求分式x^3-x^2-xy^2+y^3/-x^4+2x^2y^2-y^4的值,要过程
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x^3-x^2-xy^2+y^3/-x^4+2x^2y^2-y^4
=[(x^3+y^3)-xy(x+y)]/[-(x^2-y^2)^2]
=[(x+y)(x^2-xy+y^2)-xy(x+y)]/[-(x^2-y^2)^2]
=[x^2-2xy+y^2]/[-(x^2-y^2)^2]
=-(x-y)^2/(x-y)^2(x+y)^=-1/(x+y)^2=-1
=[(x^3+y^3)-xy(x+y)]/[-(x^2-y^2)^2]
=[(x+y)(x^2-xy+y^2)-xy(x+y)]/[-(x^2-y^2)^2]
=[x^2-2xy+y^2]/[-(x^2-y^2)^2]
=-(x-y)^2/(x-y)^2(x+y)^=-1/(x+y)^2=-1
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