已知A=5x²-3xy²+2xy,B=7xy²-2xy+x²y

題目：

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

解答：

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
再問： 可以把序號標清楚麼？謝謝！^_^
再答： 追問後就不能修改原來答案了，我就這裡修改了 (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