`a,f(x)=-3x^2+x-1+x^4-x^3-x^2+3x^4`
`=(-3x^2-x^2)+x-1+(x^4+3x^4)-x^3`
`=-4x^2+x-1+4x^4-x^3`
`⇒f(x)=4x^4-x^3-4x^2+x-1`
`g(x)=x^4+x^2-x^3+x-5+5x^3-x^2`
`=x^4+(x^2-x^2)+(-x^3+5x^3)+x-5`
`=x^4+4x^3+x-5`
`b,f(x)-g(x)=4x^4-x^3-4x^2+x-1-(x^4+4x^3+x-5)`
`=4x^4-x^3-4x^2+x-1-x^4-4x^3-x+5`
`=(4x^4-x^4)+(-x^3-4x^3)-4x^2+(x-x)+(-1+5)`
`=3x^4-5x^3-4x^2+4`
`f(x)+g(x)=4x^4-x^3-4x^2+x-1+x^4+4x^3+x-5`
`=(4x^4+x^4)+(-x^3+4x^3)-4x^2+(x+x)+(-1-5)`
`=5x^4+3x^3-4x^2+2x-6`
$\text{c, Tại x = -1}$
`⇒g(x)=(-1)^4 + 4.(-1)^3 + (-1) - 5`
`=1-4-1-5=-9`
