$\text{Câu 2:}$
`a, P(x) = 6x^2 - 5x^4 - 5x + 3 + 2x^2`
`= -5x^4 + (6x^2 + 2x^2) - 5x + 3`
`= -5x^4 + 8x^2 - 5x + 3`
`Q(x) = 5x - 6x^2 - 3 + 6x^4`
`= 6x^4 - 6x^2 + 5x - 3`
`b, M(x) = P(x) + Q(x)`
`= (-5x^4 + 8x^2 - 5x + 3) + ( 6x^4 - 6x^2 + 5x - 3)`
`= -5x^4 + 8x^2 - 5x + 3 + 6x^4 - 6x^2 + 5x - 3`
`= (-5x^4 + 6x^4) + (8x^2 - 6x^2) + (-5x + 5x) + (3-3)`
`= x^4 + 2x^2`
`c, M(x) = x^4 + 2x^2 = 0`
`⇔ x².(x²+2)=0`
\(⇔ \left[ \begin{array}{l}x^2=0\\x^2+2=0\end{array} \right.\)
\(⇔ \left[ \begin{array}{l}x=0\\x^2=-2\end{array} \right.\)
` Vì ` `x² ≥ 0 ⇒ x² = -2 (loại)`
`⇒ x=0`
$\text{⇒ x = 0 là nghiệm của đa thức M(x)}$
