15. \( \cos 304^\circ, \sin(-\frac{19\pi}{6}), tg(-521^\circ), ctg(\frac{11\pi}{3}) \)

* \( \cos 304^\circ = \cos(360^\circ - 56^\circ) = \cos 56^\circ \) * \( \sin(-\frac{19\pi}{6}) = -\sin(\frac{19\pi}{6}) = -\sin(3\pi + \frac{\pi}{6}) = \sin(\frac{\pi}{6}) \) * \( tg(-521^\circ) = -tg 521^\circ = -tg(360^\circ + 161^\circ) = -tg 161^\circ = -tg(180^\circ - 19^\circ) = tg 19^\circ \) * \( ctg(\frac{11\pi}{3}) = ctg(3\pi + \frac{2\pi}{3}) = ctg(\pi + \frac{2\pi}{3}) = ctg(\frac{2\pi}{3}) = ctg(\pi - \frac{\pi}{3}) = -ctg(\frac{\pi}{3}) \)