`1) (cos x - 2)(cos 2x - 1) = 0`
`<=>` \(\left[ \begin{array}{l}cos x - 2 = 0\\cos 2x - 1 = 0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}cos x = 2 \\cos 2x = 1\end{array} \right.\)
`<=> 2x = (π)/2 + kπ`
`<=> x = π/4 + k(π)/2` `(k ∈ ZZ)`
`2) cos 3x + cos (x + (π)/6) = 0`
`<=> cos (x + (π)/6) = -cos 3x`
`<=> cos (x + (π)/6) = cos (-3x)`
`<=>` \(\left[ \begin{array}{l}x + \frac{π}{6} = -3x + k2π\\x + \frac{π}{6} = 3x + k2π\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}4x = -\frac{π}{6} + k2π\\-2x = - \frac{π}{6} + k2π\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x = -\frac{π}{24} + k\frac{π}{2}\\x = \frac{π}{12} - kπ\end{array} \right.\) `(k ∈ ZZ)`
