$\displaystyle \cos \left (\sin ^{{-1}} \left (- \frac{x} {2} \right) \right)$ 의 값은?

• ①$\displaystyle \frac{\sqrt {1-x ^ {2}}} {2}$
• ②$\displaystyle \frac{\sqrt {1+x ^ {2}}} {2}$
• ③$\displaystyle \frac{\sqrt {4-x ^ {2}}} {2}$
• ④$\displaystyle \frac{\sqrt {4+x ^ {2}}} {2}$