\[1)\ y = \frac{1}{\cos x};\]
\[\cos x \neq 0;\]
\[x \neq \frac{\pi}{2} + \pi\text{n.}\]
\[2)\ y = \frac{2}{\sin x};\]
\[\sin x \neq 0;\]
\[x \neq \pi\text{n.}\]
\[3)\ y = tg\frac{x}{3};\]
\[\frac{x}{3} \neq \frac{\pi}{2} + \pi n;\]
\[x \neq \frac{3\pi}{2} + 3\pi n.\]
\[4)\ y = tg\ 5x;\]
\[5x \neq \frac{\pi}{2} + \pi n;\]
\[x \neq \frac{\pi}{10} + \frac{\pi n}{5}.\]