`ĐKXĐ:`
`sin 2x(cos 4x + 1) ne 0`
`<=>` \(\left[ \begin{array}{l}sin 2x \ne 0\\cos 4x \ne - 1\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}2x \ne kπ\\4x \ne π + k2π\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x \ne k\dfrac{π}{2}\\x = \dfrac{π}{4} + k\dfrac{π}{2}\end{array} \right.\)
`=> x ne k(π)/4`
`=> D = RR` \ `{k(π)/4 | k ∈ ZZ}`