곡선 $\displaystyle \mathbf{\gamma} (t) = \left (\frac{t+1} {t^2 +1}, \frac{t (t+1)} {t^2 +1} \right), 0 \leq t \leq 1$ 의 길이는?

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