2(x2-xy)-3(2x2-3xy)-2[x2-(2x2-xy+y2)] 化简

2(x²-xy)-3(2x²-3xy)-2[x²-(2x²-xy+y²)] ＝2x²-2xy-6x²+9xy-2(x²-2x²+xy-y²)=-4x²+7xy+2x²-2xy+2y²=-2x²+5xy+2y² 一个多项式加上2x
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