Упражнения 28. Отметьте на единичной окружности точку \( P_{a} \), если: \( \begin{array}{ll}\text { а) } a=\frac{\pi}{6}, a=\frac{\pi}{2}, a=\frac{3 \pi}{4} ; & \text { б) } a=\frac{\pi}{4}, a=\pi, a=-\frac{\pi}{2} ; \\ \text { в) } a=\frac{\pi}{3}, a=\frac{3 \pi}{2}, a=-\frac{\pi}{4} ; \text { г) } a=-\frac{\pi}{6}, a=2 \pi, a=\frac{5 \pi}{4} .\end{array} \)

### а) \( a=\frac{\pi}{6}, a=\frac{\pi}{2}, a=\frac{3 \pi}{4} \) 1. \( P_{\frac{\pi}{6}} \left( \frac{\sqrt{3}}{2}, \frac{1}{2} \right) \) 2. \( P_{\frac{\pi}{2}} (0, 1) \) 3. \( P_{\frac{3\pi}{4}} \left( -\frac{\sqrt{2}}{2}, \frac{\sqrt{2}}{2} \right) \) ### б) \( a=\frac{\pi}{4}, a=\pi, a=-\frac{\pi}{2} \) 1. \( P_{\frac{\pi}{4}} \left( \frac{\sqrt{2}}{2}, \frac{\sqrt{2}}{2} \right) \) 2. \( P_{\pi} (-1, 0) \) 3. \( P_{-\frac{\pi}{2}} (0, -1) \) ### в) \( a=\frac{\pi}{3}, a=\frac{3 \pi}{2}, a=-\frac{\pi}{4} \) 1. \( P_{\frac{\pi}{3}} \left( \frac{1}{2}, \frac{\sqrt{3}}{2} \right) \) 2. \( P_{\frac{3\pi}{2}} (0, -1) \) 3. \( P_{-\frac{\pi}{4}} \left( \frac{\sqrt{2}}{2}, -\frac{\sqrt{2}}{2} \right) \) ### г) \( a=-\frac{\pi}{6}, a=2\pi, a=\frac{5 \pi}{4} \) 1. \( P_{-\frac{\pi}{6}} \left( \frac{\sqrt{3}}{2}, -\frac{1}{2} \right) \) 2. \( P_{2\pi} (1, 0) \) 3. \( P_{\frac{5\pi}{4}} \left( -\frac{\sqrt{2}}{2}, -\frac{\sqrt{2}}{2} \right) \)