$sin(3cosx) = 1$$\Leftrightarrow 3cosx = \frac{\pi }{2}+k2\pi $
$\Leftrightarrow cosx = \frac{\pi }{6}+\frac{k2\pi }{3}$
Mà $-1\leq cosx \leq 1$
$\Leftrightarrow -1\leq \frac{\pi }{6}+\frac{k2\pi }{3}\leq 1$
$\rightarrow k =0.$
$cosx = \frac{\pi }{6}$
$\Rightarrow x = \pm arccos\frac{\pi }{6} +m2\pi ; m\epsilon Z.$