Giải phương trình \(\cos x + \cos 3x + 2\cos 5x = 0\).
A.\(x = \dfrac{\pi }{2} + k\pi \), \(x = \pm \dfrac{1}{5}\arccos \dfrac{{1 + \sqrt {17} }}{8} + k\pi \), \(x = \pm \dfrac{1}{5}\arccos \dfrac{{1 - \sqrt {17} }}{8} + k\pi \).
B.\(x = \ pm \dfrac{\pi }{6} + k\pi \)
C.\(x = \pm \dfrac{1}{2}\arccos \dfrac{{1 + \sqrt {15} }}{7} + k\pi \), \(x = \pm \dfrac{1}{2}\arccos \dfrac{{1 - \sqrt {15} }}{7} + k\pi \).
D.\(x = \dfrac{\pi }{2} + k\pi \), \(x = \pm \dfrac{1}{2}\arccos \dfrac{{1 + \sqrt {17} }}{8} + k\pi \), \(x = \pm \dfrac{1}{2}\arccos \dfrac{{1 - \sqrt {17} }}{8} + k\pi \).
A.\(x = \dfrac{\pi }{2} + k\pi \), \(x = \pm \dfrac{1}{5}\arccos \dfrac{{1 + \sqrt {17} }}{8} + k\pi \), \(x = \pm \dfrac{1}{5}\arccos \dfrac{{1 - \sqrt {17} }}{8} + k\pi \).
B.\(x = \ pm \dfrac{\pi }{6} + k\pi \)
C.\(x = \pm \dfrac{1}{2}\arccos \dfrac{{1 + \sqrt {15} }}{7} + k\pi \), \(x = \pm \dfrac{1}{2}\arccos \dfrac{{1 - \sqrt {15} }}{7} + k\pi \).
D.\(x = \dfrac{\pi }{2} + k\pi \), \(x = \pm \dfrac{1}{2}\arccos \dfrac{{1 + \sqrt {17} }}{8} + k\pi \), \(x = \pm \dfrac{1}{2}\arccos \dfrac{{1 - \sqrt {17} }}{8} + k\pi \).
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