삼각함수 값

$\cos \left( \pi - \theta \right) > 0$에서 $\cos \left( \pi - \theta \right) = - \cos \theta$이므로 $\cos \theta < 0$ $\sin \theta = - 3 \cos \theta$이므로 $\sin ^{2} \theta + \cos ^{2} \theta = 1$에서 $9 \cos ^{2} \theta + \cos ^{2} \theta = 1$, $10 \cos ^{2} \theta = 1$ $\displaystyle \cos ^{2} \theta = \frac{1}{10}$ $\therefore$ $\displaystyle \cos \theta = - \frac{\sqrt{10}}{10} \left( \because \cos \theta < 0 \right)$ $\therefore$ $\displaystyle \sin \theta = - 3 \times \left( - \frac{\sqrt{10}}{10} \right) = \frac{3 \sqrt{10}}{10}$