Trigonometry (10th Edition)

$\cos$ 30$^{\circ}$ = $\frac{-\sqrt3}{2}$ $\sec$ 30$^{\circ}$ = -$\frac{2\sqrt3}{3}$
We use the reference angle to determine the trigonometric functions. $\theta$$^{1} = 210^{\circ} - 180^{\circ} \theta$$^{1}$ = 30$^{\circ}$ $\cos$ 30$^{\circ}$ = $\frac{-\sqrt3}{2}$ $\sec$ 30$^{\circ}$ = $\frac{2}{-\sqrt3}$ = -$\frac{2\sqrt3}{3}$