정적분 $\displaystyle \int _0^ {\pi} \frac{x \sin ^3 x} {\cos ^2 x +1} dx$ 의 값은?

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