<=> 2.cos2x.cosx = \(2\sqrt{3}\) cos2x.sinx
<=> cos2x.cosx = \(\sqrt{3}\) cos2x.sinx
<=> cos2x.( cosx - \(\sqrt{3}\) sinx) = 0
<=> \(\left[\begin{array}{nghiempt}cos2x=0\\cosx-\sqrt{3}sinx=0\end{array}\right.\)
<=> \(\left[\begin{array}{nghiempt}2x=\frac{\pi}{2}+k\pi\\\frac{1}{2}cosx-\frac{\sqrt{3}}{2}sinx=0
\end{array}\right.\)
<=> \(\left[\begin{array}{nghiempt}x=\frac{\pi}{4}+\frac{k\pi}{2}\\sin\frac{\pi}{6}.cosx-cos\frac{\pi}{6}.sinx=0\end{array}\right.\)
<=> \(\left[\begin{array}{nghiempt}x=\frac{\pi}{4}+\frac{k\pi}{2}\\sin\left(\frac{\pi}{6}-x\right)=0\end{array}\right.\)
<=> \(\left[\begin{array}{nghiempt}x=\frac{\pi}{4}+\frac{k\pi}{2}\\x=\frac{\pi}{6}-k\pi\end{array}\right.\) (k\(\in\)Z)
