Giúp mình phần d,f,k với

$$\eqalign{
& d)\,\,y = {3 \over {{{\sin }^2}x – {{\cos }^2}x}} = – {3 \over {\cos 2x}} \cr
& DKXD:\,\,\cos 2x \ne 0 \Leftrightarrow 2x \ne {\pi \over 2} + k\pi \cr
& \Leftrightarrow x \ne {\pi \over 4} + {{k\pi } \over 2}\,\,\left( {k \in Z} \right) \cr
& \Rightarrow D = R\backslash \left\{ {{\pi \over 4} + {{k\pi } \over 2},\,k \in Z} \right\} \cr
& f)\,\,y = {2 \over {\cos x – \cos 3x}} \cr
& DKXD:\,\,\cos x \ne \cos 3x \Leftrightarrow \left[ \matrix{
3x \ne x + k2\pi \hfill \cr
3x \ne – x + k2\pi \hfill \cr} \right. \cr
& \Leftrightarrow \left[ \matrix{
2x \ne k2\pi \hfill \cr
4x \ne k2\pi \hfill \cr} \right. \Leftrightarrow \left[ \matrix{
x \ne k\pi \hfill \cr
x \ne {{k\pi } \over 2} \hfill \cr} \right. \Leftrightarrow x \ne {{k\pi } \over 2} \cr
& \Rightarrow D = R\backslash \left\{ {x \ne {{k\pi } \over 2};\,\,k \in Z} \right\} \cr
& k)\,\,y = \tan \left( {{\pi \over 2}\cos x} \right) \cr
& DKXD:\,\,{\pi \over 2}\cos x \ne {\pi \over 2} + k\pi \cr
& \Leftrightarrow \cos x \ne {1 \over 2} + k \cr
& TH1:\,\,\left| {{1 \over 2} + k} \right| > 1 \Leftrightarrow \left[ \matrix{
{1 \over 2} + k > 1 \hfill \cr
{1 \over 2} + k < - 1 \hfill \cr} \right. \cr & \Leftrightarrow \left[ \matrix{ k > {1 \over 2} \hfill \cr
k < - {3 \over 2} \hfill \cr} \right. \Rightarrow TXD:\,\,D = R \cr & TH2:\,\,\left| {{1 \over 2} + k} \right| \le 1 \Leftrightarrow \left\{ \matrix{ {1 \over 2} + k \le 1 \hfill \cr {1 \over 2} + k \ge - 1 \hfill \cr} \right. \cr & \Leftrightarrow \left\{ \matrix{ k \le {1 \over 2} \hfill \cr k \ge - {3 \over 2} \hfill \cr} \right. \Leftrightarrow - {3 \over 2} \le k \le {1 \over 2} \cr & \Rightarrow \cos x \ne {1 \over 2} + k \Rightarrow x \ne \pm \arccos \left( {{1 \over 2} + k} \right) + k2\pi \cr & \Rightarrow D = R\backslash \left\{ { \pm \arccos \left( {{1 \over 2} + k} \right) + k2\pi ;\,\,k \in Z} \right\} \cr} $$