Giải phương trình: \(1 + \cos x + \cos 2x + \cos 3x = 0 \)
A.\(\left[ \begin{array}{l}x = \frac{\pi }{2} + k2\pi \\x = \frac{\pi }{3} + \frac{{2m\pi }}{3}\end{array} \right.\,\,\,\,\left( {k,\;m \in \mathbb{Z}} \right)\)
B.\(\left[ \begin{array}{l}x = \frac{\pi }{2} + \frac{{k\pi }}{2}\\x = \frac{\pi }{3} + \frac{{2m\pi }}{3}\end{array} \right.\,\,\,\,\left( {k,\;m \in \mathbb{Z}} \right)\)              
C.\(\left[ \begin{array}{l}x = \frac{\pi }{2} + k\pi \\x = \frac{\pi }{3} + \frac{{2m\pi }}{3}\end{array} \right.\,\,\,\,\left( {k,\;m \in \mathbb{Z}} \right)\)
D.\(\left[ \begin{array}{l}x = \frac{\pi }{2} + k\pi \\x = \frac{\pi }{3} + \frac{{m\pi }}{3}\end{array} \right.\,\,\,\,\left( {k,\;m \in \mathbb{Z}} \right)\)

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