`***`Lời giải`***`
a)
`f(x)+g(x)=x^3-2x+1+2x^2-x^3+x-3`
`=(x^3-x^3)+2x^2+(-2x+x)+(-3+1)`
`=2x^2-x-2`
`f(x)-g(x)=x^3-2x+1-(2x^2-x^3+x-3)`
`=x^3-2x+1-2x^2+x^3-x+3`
`=(x^3+x^3)-2x^2+(-2x-x)+(1+3)`
`=2x^3-2x^2-3x+4`
b)
Thay `x=-1`
Ta có:`f(x)+g(x)=2.(-1)^2+1-2=1`
Thay `x=-2`
Ta có:`f(x)+g(x)=2.(-2)^2+2-2=8`