Решить уравнения: a) cos 2x = 1/2, b) tg(x/2) = -1

a) $$2x = \pm \frac{\pi}{3} + 2\pi n \implies x = \pm \frac{\pi}{6} + \pi n, n \in \mathbb{Z}$$

b) $$\frac{x}{2} = -\frac{\pi}{4} + \pi n \implies x = -\frac{\pi}{2} + 2\pi n, n \in \mathbb{Z}$$