Bài 1:
`a)`
`A+B`
`=x³+y³-2xy²+3x²y+xy-y+x-1+2x³+y³-3x²y+xy²+xy-2y+x-5`
`=(x³+2x³)+(y³+y³)+(-2xy²+xy²)+(3x²y-3x²y)+(xy+xy)+(x+x)-(y+2y)-(1+5)`
`=3x³+2y³-xy²+2xy+2x-3y-6`
`A-B`
`=x³+y³-2xy²+3x²y+xy-y+x-1-(2x³+y³-3x²y+xy²+xy-2y+x-5)`
`=x³+y³-2xy²+3x²y+xy-y+x-1-2x³-y³+3x²y-xy²-xy+2y-x+5`
`=(x³-2x³)+(y³-y³)+(-2xy²-xy²)+(3x²y+3x²y)+(xy-xy)+(x-x)+(-y+2y)+(-1+5)`
`=-x³-3xy²+6x²y+y+4`
`b)`
Thay `x=-1;y=(-2)/3` vào đa thức `A` ta được:
`A=(-1)³+((-2)/3)^3-2.(-1).((-2)/3)^2+3.(-1)². (-2)/3+(-1).(-2)/3-(-2)/3+(-1)-1`
`=-1-8/27+8/9-2+2/3+2/3-1-1`
`=-27/27-8/27+24/27-54/27+18/27+18/27-27/27-27/27`
`=-83/27`
Vậy giá trị của đa thức `A` tại `x=-1;y=(-2)/3` là `-83/27`
