Em tách ra hỏi riêng từng ý thôi em nhé
\(\begin{array}{l}
7a)\,\cos x = 0\,khong\,thoa\,\,man\,pt\\
\Rightarrow \cos x \ne 0\\
Ta\,co\,pt:\,\\
\dfrac{{{{\sin }^2}x}}{{{{\cos }^2}x}} - 2\dfrac{{\sin x\cos x}}{{{{\cos }^2}x}} - 3\dfrac{{{{\cos }^2}x}}{{{{\cos }^2}x}} = 0\\
\Leftrightarrow {\tan ^2}x - 2\tan x - 3 = 0\\
\Leftrightarrow \left[ \begin{array}{l}
\tan x = - 1\\
\tan x = 3
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
x = - \dfrac{\pi }{4} + k\pi \\
x = \arctan 3 + k\pi
\end{array} \right.
\end{array}\)
