\[1)\ y = \sin{2x};\]
\[D(x) = ( - \infty;\ + \infty).\]
\[2)\ y = \cos\frac{x}{2};\]
\[D(x) = ( - \infty;\ + \infty).\]
\[3)\ y = \cos\frac{1}{x};\]
\[x \neq 0;\]
\[D(x) = ( - \infty;\ 0) \cup (0;\ + \infty).\]
\[4)\ y = \sin\frac{2}{x};\]
\[x \neq 0;\]
\[D(x) = ( - \infty;\ 0) \cup (0;\ + \infty).\]
\[5)\ y = \sin\sqrt{x};\]
\[x \geq 0;\]
\[D(x) = \lbrack 0;\ + \infty).\]
\[6)\ y = \cos\sqrt{\frac{x - 1}{x + 1}};\]
\[\frac{x - 1}{x + 1} \geq 0;\]
\[x < - 1;\]
\[x \geq 1;\]
\[D(x) = ( - \infty;\ - 1) \cup \lbrack 1;\ + \infty).\]