Ответ:
\[B\left( - \frac{\sqrt{3}}{2};\frac{1}{2} \right);\ \ C\left( - \frac{\sqrt{3}}{2};\ - \frac{1}{2} \right);\ \ \]
\[\alpha = \frac{5\pi}{6};\ \ \beta = \frac{7\pi}{6}.\]
\[B\left( - \frac{\sqrt{3}}{2};\frac{1}{2} \right);\ \ C\left( - \frac{\sqrt{3}}{2};\ - \frac{1}{2} \right);\ \ \]
\[\alpha = \frac{5\pi}{6};\ \ \beta = \frac{7\pi}{6}.\]