Монгол Бодлогын Сан

$(\tg^2x+3)\cos x=3\Leftrightarrow \left\{ % \begin{array}{l} t=\cos x\ne 0 \\ \fbox{a}t^2-\fbox{b}t+1=0 \\ \end{array} % \right. \Leftrightarrow \left[ % \begin{array}{l} \cos x=\fbox{c}\\ \cos x=\dfrac1{\fbox{d}} \\ \end{array} % \right. \Leftrightarrow\\ \Leftrightarrow \left[ % \begin{array}{l} x=\fbox{e}k\pi \\ x=\pm\dfrac{\pi}{\fbox{f}}+2n\pi, k,n\in \mathbb Z. \\ \end{array} % \right.$