극곡선 $\displaystyle r= \sin \theta +2 \cos \theta (0 \leq \theta \leq \frac{\pi} {2})$ 의 길이는?

• ①$\displaystyle \frac{\sqrt {5}} {2} \pi$
• ②$\displaystyle \frac{\sqrt {6}} {2} \pi$
• ③$\displaystyle \frac{\sqrt {7}} {2} \pi$
• ④$\sqrt {2} \pi$
• ⑤$\displaystyle \frac{3} {2} \pi$