x(x+y)(x-y)-x(x+y)^2
=x(x+y)〔(x-y)-(x+y)〕
=x(x+y)(-2y)
=-2xy(x+y)
把x+y=1,xy=-1/2代入-2xy(x+y)可得：
-2xy(x+y)
=-2*(-1/2)*1
=1
已知：x+y=1,xy=-1/2,则x(x+y)(x-y)-x(x+y)^2=
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