$\displaystyle \frac{1} {\pi} \int _ {0} ^ {\frac{3} {4}} {} \frac{dx} {\sqrt {x (1-x)}}$ 의 값은?

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