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

• ①$\displaystyle \frac{1} {12}$
• ②$\displaystyle \frac{1} {6}$
• ③$\displaystyle \frac{1} {3}$
• ④$\displaystyle \frac{1} {2}$