`cos 2x - cos 8x + cos 6x = 1`
`<=> (cos 2x + cos 6x) - (1 + cos 8x) = 0`
`<=> 2cos 4x.cos 2x - 2cos^2 4x = 0`
`<=> 2cos 4x(cos 2x - cos 4x) = 0`
`<=> cos 4x(cos 2x - 2cos^2 2x + 1) = 0`
`<=> cos 4x(cos 2x - 1)(2cos 2x + 1) = 0`
`<=>` \(\left[ \begin{array}{l}cos 4x = 0\\cos 2x - 1 = 0\\2cos 2x + 1 = 0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x = \dfrac{π}{8} + k\dfrac{π}{2}\\x = kπ\\x = ±\dfrac{π}{3} + kπ\end{array} \right.\) `(k ∈ ZZ)`
