반각 코사인

$\displaystyle \cos \frac{\theta}{2} = \frac{3}{4}$일 때, $\cos \theta$의 값은? $\displaystyle \left( \begin{array}{l} \begin{matrix} \\ \end{matrix} \end{array} \right.$단, $\displaystyle \left. 0 < \theta < \frac{\pi}{2} \right.$$\displaystyle \begin{matrix} \\ \end{matrix} )$ [2점] ①$\displaystyle \frac{1}{5}$②$\displaystyle \frac{1}{6}$③$\displaystyle \frac{1}{7}$④$\displaystyle \frac{1}{8}$⑤$\displaystyle \frac{1}{9}$