20. \( \cos(-235^\circ), \sin(\frac{13\pi}{6}), ctg 247^\circ, tg(-\frac{11\pi}{5}) \)

* \( \cos(-235^\circ) = \cos 235^\circ = \cos(180^\circ + 55^\circ) = -\cos 55^\circ \) * \( \sin(\frac{13\pi}{6}) = \sin(2\pi + \frac{\pi}{6}) = \sin(\frac{\pi}{6}) \) * \( ctg 247^\circ = ctg(180^\circ + 67^\circ) = ctg 67^\circ \) * \( tg(-\frac{11\pi}{5}) = -tg(\frac{11\pi}{5}) = -tg(2\pi + \frac{\pi}{5}) = -tg(\frac{\pi}{5}) \)