[tex]\displaystyle cos5x < \frac{1}{2}\\\\ \frac{\pi }{3}+2\pi n < 5x < \frac{5\pi }{3}+2\pi n; n \in Z\\\\\frac{\pi }{15}+\frac{2}{5}\pi n < x < \frac{\pi }{3}+\frac{2}{5}\pi n; n \in Z[/tex]