21.13. в) arccos \(\frac{\sqrt{3}}{2}\); г) arccos \(\frac{1}{2}\);

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