13. \( \cos 423^\circ, \sin(-\frac{11\pi}{4}), ctg (-235^\circ), ctg(\frac{19\pi}{6}) \)

* \( \cos 423^\circ = \cos(360^\circ + 63^\circ) = \cos 63^\circ \) * \( \sin(-\frac{11\pi}{4}) = -\sin(\frac{11\pi}{4}) = -\sin(2\pi + \frac{3\pi}{4}) = -\sin(\frac{3\pi}{4}) = -\sin(\pi - \frac{\pi}{4}) = -\sin(\frac{\pi}{4}) \) * \( ctg (-235^\circ) = -ctg 235^\circ = -ctg(180^\circ + 55^\circ) = -ctg 55^\circ \) * \( ctg(\frac{19\pi}{6}) = ctg(3\pi + \frac{\pi}{6}) = ctg(\pi + \frac{\pi}{6}) = ctg(\frac{\pi}{6}) \)