an increasing function of \(x\)?

Draw a rough sketch-graph of \(y = 3 \cos x - \cos 3x\) for values of \(x\) from \(0\) to \(\pi\), indicating clearly any maximum or minimum points.

Prove that
\[\begin{equation*}
\frac{d}{dx} (3 \cos x - \cos 3x)= 6 \sin x \cos 2x.
\end{equation*}\]
For what two ranges of values of \(x\) between \(0\) and \(\pi\) is
\[\begin{equation*}
3 \cos x - \cos 3x
\end{equation*}\]

an increasing function of \(x\)?

Draw a rough sketch-graph of \(y = 3 \cos x - \cos 3x\) for values of \(x\) from \(0\) to \(\pi\), indicating clearly any maximum or minimum points.