已知A=-x²+2xy-y²,B=xy-3x²,C=2y²-xy.求2A-B-3C的值.
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已知A=-x²+2xy-y²,B=xy-3x²,C=2y²-xy.求2A-B-3C的值.
已知A=-x²+2xy-y²,B=xy-3x²,C=2y²-xy.求2A-B-3C的值.
已知A=-x²+2xy-y²,B=xy-3x²,C=2y²-xy.求2A-B-3C的值.
2A-B-3C=2(-x2+2xy-y2)-(xy-3x2)-3(2y2-xy)
=-2x2+4xy-2y2-xy+3x2-6y2+3xy
=x2-8y2+7xy