$\displaystyle \lim _ {x \rightarrow 0} \frac{\cos x- \cos \left (x+x^ {2} \right)} {x^ {3}}$ 의 값은?

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