已知:A=-x²+2xy-y²,Bxy-3x²,C=2y²-xy,求A-2B+3C

问题描述:

已知:A=-x²+2xy-y²,Bxy-3x²,C=2y²-xy,求A-2B+3C

A-2B+3C
=(-x²+2xy-y²)-2(xy-3x²)+3(2y²-xy)
=-x²+2xy-y²-2xy+6x²+6y²-3xy
=5x²-3xy+5y²