곡선 $\displaystyle y= \ln \left (\sin \left (x \right) \right) \left (\frac{\pi} {6} \leq x \leq \frac{\pi} {2} \right)$ 의 길이를 구하면?

• ①$\ln \left (2- \sqrt {3} \right)$
• ②$\ln \left (2+ \sqrt {3} \right)$
• ③$\ln \left (\sqrt {2} -1 \right)$
• ④$1$
• ⑤$\ln \left (\sqrt {2} +1 \right)$