`text{Ta có}`
`cot^2 x >= 0 -> 1 + cot^2 x > 0`
`ĐKXĐ:`
\(\left\{ \begin{array}{l}sin x \ne 0\\1 - sin 3x > 0\end{array} \right.\)
`->` \(\left\{ \begin{array}{l}x \ne k\pi\\sin 3x < 1 \(text{thoả mãn})\end{array} \right.\) `(k in ZZ)`
`->` \(\left\{ \begin{array}{l}x \ne k\pi\\3x \ne \dfrac{\pi}{2} + k2\pi\end{array} \right.\) `(k in ZZ)`
`->` \(\left\{ \begin{array}{l}x \ne k\pi\\x \ne \dfrac{\pi}{6} + \dfrac{k2\pi}{3}\end{array} \right.\) `(k in ZZ)`
`-> D = RR \\ {kpi; (pi)/6 + (k2pi)/3 | k in ZZ}`
