정적분 $\displaystyle \int _ {0} ^ {1} {\frac{1} {\left (x^2 +1 \right) ^2}} dx$ 를 구하면?

• ①$\displaystyle \frac{1} {2} + \frac{\pi} {6}$
• ②$\displaystyle \frac{1} {3} + \frac{\pi} {7}$
• ③$\displaystyle \frac{1} {4} + \frac{\pi} {8}$
• ④$\displaystyle \frac{1} {5} + \frac{\pi} {9}$
• ⑤$\displaystyle \frac{1} {6} + \frac{\pi} {10}$