\[\boxed{\mathbf{693}\mathbf{.}}\]
\[1)\ y = \frac{1}{\cos x}\]
\[\cos x \neq 0\]
\[x \neq \arccos 0 + \pi n = \frac{\pi}{2} + \pi n\]
\[Ответ:\ \ x \neq \frac{\pi}{2} + \pi n.\]
\[2)\ y = \frac{2}{\sin x}\]
\[\sin x \neq 0\]
\[x \neq \arcsin 0 + \pi n = \pi n\]
\[Ответ:\ \ x \neq \pi n.\]
\[3)\ y = tg\frac{x}{3}\]
\[\frac{x}{3} \neq \frac{\pi}{2} + \pi n\]
\[x \neq 3 \bullet \left( \frac{\pi}{2} + \pi n \right) = \frac{3\pi}{2} + 3\pi n\]
\[Ответ:\ \ x \neq \frac{3\pi}{2} + 3\pi n.\]
\[4)\ y = tg\ 5x\]
\[5x \neq \frac{\pi}{2} + \pi n\]
\[x \neq \frac{1}{5} \bullet \left( \frac{\pi}{2} + \pi n \right) = \frac{\pi}{10} + \frac{\text{πn}}{5}\]
\[Ответ:\ \ x \neq \frac{\pi}{10} + \frac{\text{πn}}{5}.\]