Решение:
x = \frac{\pi}{3} + \frac{2k\pi}{3} " alt=" \cos(x) + \cos(2x) = 0 \\ 2 \cos( \frac{3x}{2} ) \cos( - \frac{x}{2} ) = 0 \\ 2 \cos( \frac{3x}{2} ) \cos( \frac{x}{2} ) = 0 \\ \cos( \frac{3x}{2} ) \cos( \frac{x}{2} ) = 0 \\ \\ \cos( \frac{3x}{2} ) = 0 \\ \cos( \frac{x}{2} ) = 0 \\ \\ 1) \\ \frac{3x}{2} = \frac{\pi}{2} + k\pi \\ x = \frac{\pi}{3} + \frac{2k\pi}{3} \\ \\ 2) \\ \frac{x}{2} = \frac{\pi}{2} + k\pi \\ x = \pi + 2k\pi \\ \\ = > x = \frac{\pi}{3} + \frac{2k\pi}{3} " align="absmiddle" class="latex-formula">