Ta có: \(y' = 3x^2- 6x\).
\(\begin{array}{l}
a)\,\,y' > 0\\
\Leftrightarrow 3{x^2} - 6x > 0\\
\Leftrightarrow \left[ \begin{array}{l}
x > 2\\
x < 0
\end{array} \right.\\
\Rightarrow S = \left( { - \infty ;0} \right) \cup \left( {2; + \infty } \right)\\
b)\,\,y' < 3\\
\Leftrightarrow 3{x^2} - 6x < 3\\
\Leftrightarrow 1 - \sqrt 2 < x < 1 + \sqrt 2 \\
\Rightarrow S = \left( {1 - \sqrt 2 ;1 + \sqrt 2 } \right)
\end{array}\)
