021.17. a) cos (2 arccos \(\frac{1}{2}\) - 3 arccos 0 - arccos (\(-\frac{1}{2}\))); б) \(\frac{1}{3}\) (arccos \(\frac{1}{3}\) + arccos (\(-\frac{1}{3}\)));

a) cos (2 arccos \(\frac{1}{2}\) - 3 arccos 0 - arccos (\(-\frac{1}{2}\))) = cos (2 * \(\frac{\pi}{3}\) - 3 * \(\frac{\pi}{2}\) - \(\frac{2\pi}{3}\)) = cos (\(\frac{2\pi}{3}\) - \(\frac{3\pi}{2}\) - \(\frac{2\pi}{3}\)) = cos ( -\(\frac{3\pi}{2}\)) = cos (\(\frac{\pi}{2}\)) = 0. б) \(\frac{1}{3}\) (arccos \(\frac{1}{3}\) + arccos (\(-\frac{1}{3}\))) = \(\frac{1}{3}\) (arccos \(\frac{1}{3}\) + \(\pi\) - arccos (\(\frac{1}{3}\))) = \(\frac{1}{3}\) * \(\pi\) = \(\frac{\pi}{3}\).