`text{Bài 2}`
`cos 6x - cos 4x + cos 2x = 1`
`-> (cos 6x - cos 4x) + 1 - 2sin^2 x - 1 = 0`
`-> -2sin 5x.sin x - 2sin^2 x = 0`
`-> sin x(sin 5x + sin x) = 0`
`-> sin x.2.sin 3x.cos 2x = 0`
`-> sin x.sin 3x.cos 2x = 0`
`->` \(\left[ \begin{array}{l}sin x = 0\\sin 3x = 0\\cos 2x = 0\end{array} \right.\)
`->` \(\left[ \begin{array}{l}x = k\pi\\x = k\dfrac{\pi}{3}\\x = \dfrac{\pi}{4} + k\dfrac{\pi}{2}\end{array} \right.\)
`->` \(\left[ \begin{array}{l}x = k\dfrac{\pi}{3}\\x = \dfrac{\pi}{4} + k\dfrac{\pi}{2}\end{array} \right.\) `(k in ZZ)`
