급수 $\displaystyle \sum _ {n=0} ^ {\infty} \frac{3^ {n}} {4^ {n} (2 n)!}$ 의 합은?

• ①$\displaystyle {\cos} \left (\frac{\sqrt {3}} {2} \right)$
• ②$\displaystyle {\sin} \left (\frac{\sqrt {3}} {2} \right)$
• ③$\displaystyle \frac{e^ {\frac{\sqrt {3}} {2}} -e^ {- \frac{\sqrt {3}} {2}}} {4}$
• ④$\displaystyle \frac{e^ {\frac{\sqrt {3}} {2}} +e^ {- \frac{\sqrt {3}} {2}}} {4}$
• ⑤$\displaystyle \frac{e^ {\frac{\sqrt {3}} {2}} +e^ {- \frac{\sqrt {3}} {2}}} {2}$