\(M=\dfrac{3x^2}{3\left(x^4+x^2+1\right)}=\dfrac{x^4+x^2+1-\left(x^4-2x^2+1\right)}{3\left(x^4+x^2+1\right)}=\dfrac{1}{3}-\dfrac{\left(x^2-1\right)^2}{3\left(x^4+x^2+1\right)}\le\dfrac{1}{3}\)
\(M_{max}=\dfrac{1}{3}\) khi \(x^2-1=0\Leftrightarrow x=\pm1\)