극방정식 $r= \theta ^2$ ($0 \leq \theta \leq 2 \pi$)로 나타나는 곡선의 길이는?

• ①$\displaystyle \frac{4} {3} \left \{\left (\pi +1 \right) ^ {1/2} -1 \right.$
• ②$\displaystyle \frac{4} {3} \left \{\left (\pi ^2 +1 \right) ^ {3/2} -1 \right.$
• ③$\displaystyle \frac{8} {3} \left \{\left (\pi +1 \right) ^ {1/2} -1 \right.$
• ④$\displaystyle \frac{8} {3} \left \{\left (\pi ^2 +1 \right) ^ {3/2} -1 \right.$
• ⑤$\displaystyle \frac{8} {3} \left \{\left (\pi +1 \right) ^ {3/2} -1 \right.$