pt <=> $2\sqrt{2}[\frac{1}{2}cos(\frac{x}{5}-\frac{\pi}{12})-\frac{\sqrt{3}}{2}sin(\frac{x}{5}-\frac{\pi}{12})]=2[sin(\frac{x}{5}+\frac{2\pi}{3})-sin(\frac{3x}{5}+\frac{\pi}{6})]$<=> $2\sqrt{2}[cos\frac{\pi}{3}.cos(\frac{x}{5}-\frac{\pi}{12})-sin\frac{\pi}{3}.sin(\frac{x}{5}-\frac{\pi}{12})]=4.cos(\frac{2x}{5}+\frac{5\pi}{12}).sin(\frac{-x}{5}+\frac{\pi}{4})$
<=> $2\sqrt{2}.cos[\frac{\pi}{3}+(\frac{x}{5}-\frac{\pi}{12})]=4.cos(\frac{2x}{5}+\frac{5\pi}{12}).cos[\frac{\pi}{2}-(\frac{-x}{5}+\frac{\pi}{4})]$
<=> $2\sqrt{2}.cos(\frac{x}{5}+\frac{\pi}{4})=4.cos(\frac{2x}{5}+\frac{5\pi}{12}).cos(\frac{x}{5}+\frac{\pi}{4})$
<=> $cos(\frac{x}{5}+\frac{\pi}{4})[1-\sqrt{2}.cos(\frac{2x}{5}+\frac{5\pi}{12})]=0$
Đến đây bạn làm nốt nhé
