第13章 13.11

$\sin\left(x+\frac{\pi}{3}\right)+2\sin\left(x-\frac{\pi}{3}\right)=0$

$\sin x \cdot \cos\frac{\pi}{3} + \cos x \cdot \sin\frac{\pi}{3} + 2\sin x \cdot \cos\frac{\pi}{3} - 2\cos x \cdot \sin\frac{\pi}{3} = 3\sin x \cdot \cos\frac{\pi}{3} - \cos x \cdot \sin\frac{\pi}{3} = \frac{3}{2}\sin x - \frac{\sqrt{3}}{2}\cos x = 0$

$3\sin x - \sqrt{3}\cos x = 0 \qquad 2\sqrt{3}\sin(x-\theta) = 0 \qquad \tan\theta = \frac{1}{\sqrt{3}} \qquad \theta = \frac{\pi}{6}, \frac{7}{6}\pi$

これから $x = \dfrac{\pi}{6}, \dfrac{7}{6}\pi$