극곡선 $r= \theta ^2 (0 \leq \theta \leq 2 \pi)$ 의 길이는?

• ①$\displaystyle \frac{8} {3} " {" (\pi ^2 +4) ^ {\frac{3} {2}} -1"} "$
• ②$\displaystyle \frac{4} {3} " {" (\pi ^ {2} +1) ^ {\frac{3} {2}} -1"} "$
• ③$\displaystyle \frac{8} {3} " {" (\pi ^ {2} +2) ^ {\frac{3} {2}} -1"} "$
• ④$\displaystyle \frac{1} {3} " {" (\pi ^ {2} +4) ^ {\frac{3} {2}} -1"} "$
• ⑤$\displaystyle \frac{8} {3} " {" (\pi ^ {2} +1) ^ {\frac{3} {2}} -1"} "$