已知A=5x²-3xy²+2xy,B=7xy²-2xy+x²y

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已知A=5x²-3xy²+2xy,B=7xy²-2xy+x²y
（1）求A-2B
（2）2A+B+C=0,求C

A-2B=5x²-3xy²+2xy-2(7xy²-2xy+x²y)
=5x²-3xy²+2xy-14xy²+4xy-2x²y
=5x²-17xy²+6xy-2x²y
2A+B+C=0
C=-2A-B
=-2(5x²-3xy²+2xy)-(7xy²-2xy+x²y)
=-10x²+6xy²-4xy-7xy²+2xy-x²y
=-10x²-2xy-xy²-x²y
再问：可以把序号标清楚么？谢谢！^_^
再答：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

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