Đáp án:
$\left[\begin{array}{l}x = \dfrac{2\pi}{3} + k2\pi\\ x = -\dfrac{2\pi}{3} + k2\pi\end{array}\right. \quad (k\in \Bbb Z)$
Giải thích các bước giải:
$(1 +2\cos x)(3-\cos x) = 0$
$\Leftrightarrow \left[\begin{array}{l}1 + 2\cos x = 0\\3 - \cos x = 0\end{array}\right.$
$\Leftrightarrow \left[\begin{array}{l}\cos x = -\dfrac{1}{2}\\\cos x = 3 \quad \text{(vô nghiệm)}\end{array}\right.$
$\Leftrightarrow \left[\begin{array}{l}x = \dfrac{2\pi}{3} + k2\pi\\ x = -\dfrac{2\pi}{3} + k2\pi\end{array}\right. \quad (k\in \Bbb Z)$
