21.15. a) arccos (-1) + arccos 0; б) arccos \(\frac{1}{2}\) - arccos \(\frac{\sqrt{3}}{2}\); в) arccos (\(-\frac{\sqrt{2}}{2}\)) + arccos \(\frac{\sqrt{2}}{2}\); г) arccos (\(-\frac{1}{2}\)) - arccos \(\frac{1}{2}\);

a) arccos (-1) + arccos 0 = \(\pi + \frac{\pi}{2} = \frac{3\pi}{2}\). б) arccos \(\frac{1}{2}\) - arccos \(\frac{\sqrt{3}}{2}\) = \(\frac{\pi}{3} - \frac{\pi}{6} = \frac{\pi}{6}\). в) arccos (\(-\frac{\sqrt{2}}{2}\)) + arccos \(\frac{\sqrt{2}}{2}\) = \(\frac{3\pi}{4} + \frac{\pi}{4} = \pi\). г) arccos (\(-\frac{1}{2}\)) - arccos \(\frac{1}{2}\) = \(\frac{2\pi}{3} - \frac{\pi}{3} = \frac{\pi}{3}\).