$\displaystyle \int _ {0} ^ {2 \pi} {\frac{\cos ^ {2} 3 \theta} {5-4 \cos 2 \theta}} d \theta$ 의 값은?

• ①$\displaystyle \frac{3} {4} \pi$
• ②$\displaystyle \frac{3} {8} \pi$
• ③$\displaystyle \frac{3} {16} \pi$
• ④$\displaystyle \frac{3} {32} \pi$