\(y'=3mx^2-2\)
Để \(y'< 0\) \(\forall x\Rightarrow\max\limits_{x\in R}g\left(x\right)=3mx^2-2< 0\)
\(g'\left(x\right)=6mx=0\Rightarrow x=0\)
\(\Rightarrow\left\{{}\begin{matrix}m\le0\\g\left(0\right)< 0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}m\le0\\-2< 0\end{matrix}\right.\) \(\Rightarrow m\le0\)