2 câu trả lời
$$\eqalign{ & \cos 2x + 2\cos x + 1 = 0 \cr & \Leftrightarrow 2{\cos ^2}x + 2\cos x = 0 \cr & \Leftrightarrow 2\cos x\left( {\cos x + 1} \right) = 0 \cr & \Leftrightarrow \left[ \matrix{ \cos x = 0 \hfill \cr \cos x = - 1 \hfill \cr} \right. \Leftrightarrow \left[ \matrix{ x = {\pi \over 2} + k\pi \hfill \cr x = \pi + k2\pi \hfill \cr} \right.\,\,\left( {k \in Z} \right) \cr} $$
