`a) P=-x³y-xy+x²+4x³y+2xy+1`
`=3x³y+xy+x²+1`
`Q=x³y-8xy-5+2x³y+9x²+4-10x²`
`=>3x³y-8xy-x²-1`
`b)A=P+Q`
`3x³y+xy+x²+1+3x³y-8xy-x²-1`
`=>6x³y+7xy`
`B=P-Q`
`=>3x³y+xy+x²+1-3x³y+8xy+x²+1`
`=>9xy+2x²+2`
thay `x=1;y=-1 vào A `ta được:
`A=6.1³.(-1)+7.1.(-1)`
`=>-13`