* $y=\sqrt{\sin 3 x}$

Hướng dẫn:

Hàm số $f(x)=\sqrt{\sin 3 x}$ xác định 

$$
\begin{array}{ll}
\Leftrightarrow & 3 x \in[k 2 \pi ; \pi+k 2 \pi] \\
\Leftrightarrow & x \in\left[\frac{k 2 \pi}{3} ; \frac{\pi}{3}+\frac{k 2 \pi}{3}\right] .
\end{array}
$$

TXĐ: $D=\left[\frac{k2\pi}{3} ; \frac{\pi}{3}+\frac{k2\pi}{3}\right], h \in \mathbb{Z}$.