(i)[2]
Determine an expression for $\frac{dy}{dx}$.
(ii)[8]
After first writing $\frac{dy}{dx}$ as $R \cos(2x + \alpha)$, where $R > 0$ and $0 < \alpha < \tfrac{1}{2}\pi$, determine the $x$-coordinates of $P$ and $Q$, giving answers correct to 4 significant figures.