21.14. a) arccos \(\frac{\sqrt{2}}{2}\); б) arccos \(\frac{\sqrt{3}}{2}\); в) arccos (-1); г) arccos (\(-\frac{1}{2}\));

a) arccos \(\frac{\sqrt{2}}{2}\) = \(\frac{\pi}{4}\), так как cos \(\frac{\pi}{4}\) = \(\frac{\sqrt{2}}{2}\). б) arccos \(\frac{\sqrt{3}}{2}\) = \(\frac{\pi}{6}\), так как cos \(\frac{\pi}{6}\) = \(\frac{\sqrt{3}}{2}\). в) arccos (-1) = \(\pi\), так как cos \(\pi\) = -1. г) arccos (\(-\frac{1}{2}\)) = \(\frac{2\pi}{3}\), так как cos \(\frac{2\pi}{3}\) = -\(\frac{1}{2}\).