A-2B=5x²-3xy²+2xy-2(7xy²-2xy+x²y)=5x²-3xy²+2xy-14xy²+4xy-2x²y=5x²-17xy²+6xy-2x²y2A+B+C=0C=-2A-B=-2(5x²-3xy²+2xy)-(7xy²-2xy+x²...可以把序号标清楚么？谢谢！^_^追问后就不能修改原来答案了，我就这里修改了(1).A-2B=5x²-3xy²+2xy-2(7xy²-2xy+x²y) =5x²-3xy²+2xy-14xy²+4xy-2x²y =5x²-17xy²+6xy-2x²y(2).2A+B+C=0 => C=-2A-B=-2(5x²-3xy²+2xy)-(7xy²-2xy+x²y)=-10x²+6xy²-4xy-7xy²+2xy-x²y =-10x²-2xy-xy²-x²y