\begin{cases} cos3x = 0\\cos2x = 0 \end{cases} \begin{cases} 3x = +- \frac{\pi}{2} + 2\pi n\\2x = +- \frac{\pi}{2} + 2\pi k\end{cases}\\ \begin{cases} x = +-\frac{\pi}{6} + \frac{2\pi}{3}n\\x = +-\frac{\pi}{4} + \pi k\end{cases}" alt="cosx + cos 5x = 0\\ 2cos3x \cdot cos(-2x) = 0\\ <=> \begin{cases} cos3x = 0\\cos2x = 0 \end{cases} \begin{cases} 3x = +- \frac{\pi}{2} + 2\pi n\\2x = +- \frac{\pi}{2} + 2\pi k\end{cases}\\ \begin{cases} x = +-\frac{\pi}{6} + \frac{2\pi}{3}n\\x = +-\frac{\pi}{4} + \pi k\end{cases}" align="absmiddle" class="latex-formula">