\[y = \cos x;\ \ \ x \in \lbrack 0;\ 3\pi\rbrack.\]
\[1)\ \cos x = 0:\]
\[\ x_{1} = \frac{\pi}{2},\ \ \ x_{2} = \frac{3\pi}{2},\ \ \ x_{3} = \frac{5\pi}{2}.\]
\[\cos x = 1:\]
\[x_{1} = 0,\ \ \ x_{2} = 2\pi.\]
\[\cos x = - 1:\ \]
\[x_{1} = \pi,\ \ \ x_{2} = 3\pi.\]
\[2)\ \cos x > 0:\]
\[0 \leq x < \frac{\pi}{2}\]
\[\frac{3\pi}{2} < x < \frac{5\pi}{2}.\]
\[3)\ \cos x < 0:\ \]
\[\frac{\pi}{2} < x < \frac{3\pi}{2}\]
\[\frac{5\pi}{2} < x \leq 3\pi.\]