정적분 $\displaystyle \int _ {0} ^ {1} {\tanh ^ {3}} x dx$ 를 구하면?

• ①$\displaystyle \ln \left (\cosh 1 \right) - \frac{\tanh ^2 1} {2}$
• ②$\displaystyle \ln \left (\cosh 1 \right) + \frac{\tanh ^2 1} {2}$
• ③$\displaystyle \frac{\ln \left (\cosh 1 \right)} {2} - \frac{\tanh ^2 1} {3}$
• ④$\displaystyle \frac{\ln \left (\cosh 1 \right)} {2} + \frac{\tanh ^2 1} {3}$
• ⑤$\displaystyle \ln \left (\cosh 1 \right) - \frac{\tanh ^2 1} {3}$