a) $y = \tan\left(3x - \dfrac{7\pi}{12}\right) + \cos4x$
$y$ xác định $\Leftrightarrow \cos\left(3x - \dfrac{7\pi}{12}\right) \ne 0$
$\Leftrightarrow 3x - \dfrac{7\pi}{12} \ne \dfrac{\pi}{2} + k\pi$
$\Leftrightarrow x \ne \dfrac{13\pi}{36} + k\dfrac{\pi}{3}$
$\Rightarrow TXD: D = \Bbb R \backslash\left\{\dfrac{13\pi}{36} + k\dfrac{\pi}{3}\, | \, k \in \Bbb Z\right\}$
b) $y = \dfrac{3}{4}\cot\left(\dfrac{3}{4}x + 2\right)$
$y$ xác định $\Leftrightarrow \sin\left(\dfrac{3}{4}x + 2\right) \ne 0$
$\Leftrightarrow \dfrac{3}{4}x + 2 \ne k\pi$
$\Leftrightarrow x \ne -\dfrac{8}{3} + k\dfrac{4\pi}{3}$
$\Rightarrow TXD: D = \Bbb R \backslash\left\{-\dfrac{8}{3} + k\dfrac{4\pi}{3}\,|\,k \in \Bbb Z \right\}$
