[tex]\bf cos\left( \frac{\pi }{6}x \right)=0\implies cos^{-1}\left[ cos\left( \frac{\pi }{6}x \right) \right]=cos^{-1}(0)
\\\\\\
\cfrac{\pi x}{6}=cos^{-1}(0)\implies \cfrac{\pi x}{6}=
\begin{cases}
\frac{\pi }{2}\\\\
\frac{3\pi }{2}
\end{cases}\\\\
-------------------------------[/tex]
[tex]\bf \cfrac{\pi x}{6}=\cfrac{\pi }{2}\implies \pi x=\cfrac{6\pi }{2}\implies x=\cfrac{6\pi }{2\pi }
\implies \measuredangle x=\stackrel{radians}{3}\\\\
-------------------------------\\\\
\cfrac{\pi x}{6}=\cfrac{3\pi }{2}\implies\pi x=\cfrac{18\pi }{2}\implies x=\cfrac{18\pi }{2\pi }\implies \measuredangle x=\stackrel{radians}{9}[/tex]
