\[1)\ y = \sqrt{\sin x + 1};\]
\[\sin x + 1 \geq 0;\]
\[\sin x \geq - 1;\]
\[x \in R.\]
\[2)\ y = \sqrt{\cos x - 1};\]
\[\cos x - 1 \geq 0;\]
\[\cos x \geq 1;\]
\[x = 2\pi n.\]
\[3)\ y = \lg{\sin x};\]
\[\sin x > 0;\]
\[2\pi n < x < \pi + 2\pi\text{n.}\]
\[4)\ y = \sqrt{2\cos x - 1};\]
\[2\cos x - 1 \geq 0;\]
\[2\cos x \geq 1;\]
\[\cos x \geq \frac{1}{2};\]
\[- \frac{\pi}{3} + 2\pi n \leq x \leq \frac{\pi}{3} + 2\pi\text{n.}\]
\[5)\ y = \sqrt{1 - 2\sin x};\]
\[1 - 2\sin x \geq 0;\]
\[2\sin x \leq 1;\]
\[\sin x \leq \frac{1}{2};\]
\[- \frac{7\pi}{6} + 2\pi n \leq x \leq \frac{\pi}{6} + 2\pi\text{n.}\]
\[6)\ y = \ln{\cos x};\]
\[\cos x > 0;\]
\[- \frac{\pi}{2} + 2\pi n < x < \frac{\pi}{2} + 2\pi\text{n.}\]