곡선 $\displaystyle y= \frac{2} {3} \sqrt {\left (x^2 +1 \right) ^3}$ ($0 \leq x \leq 1$)의 길이는?

• ①$\displaystyle \frac{7} {6}$
• ②$\displaystyle \frac{4} {3}$
• ③$\displaystyle \frac{3} {2}$
• ④$\displaystyle \frac{5} {3}$
• ⑤$\displaystyle \frac{11} {6}$