If \(\displaystyle \int_{0}^\dfrac{\pi}{3} \cos^4 x \, dx = a\pi + b\sqrt{3} \), where \( a \) and \( b \) are rational numbers, then \( 9a + 8b \) is equal to:
\( 2 \)
\( 1 \)
\( \dfrac{3}{2} \)
\( 3 \)