`a,` `A(x)=5x^4-5+6x^3+x^4-5x-12`
`=(5x^4+x^4)+6x^3-5x-(12+5)`
`=6x^4+6x^3-5x-17`
`B(x)=8x^4+2x^3-2x^4+4x^3-5x-15-2x^2`
`=(8x^4-2x^4)+(2x^3+4x^3)-2x^2-5x-15`
`=6x^4+6x^3-2x^2-5x-15`
`b,`
`A(x)=6x^4+6x^3` `-5x-17`
`-`
`B(x)=6x^4+6x^3-2x^2-5x-15`
`C(x)=A(x)-B(x)=` `2x^2` `-2`
`⇒C(x)=2x^2-2`
`C(x)=0⇔2x^2-2=0`
`⇒2x^2=2`
`⇒x^2=1`
`⇒x=±1`
Vậy `C(x)` có `2` nghiệm: `±1`