## Precalculus: Mathematics for Calculus, 7th Edition

Exact value of $\cos 150^{\circ}$ = - $\frac{\sqrt 3}{2}$
To find exact value of $\cos 150^{\circ}$, let's find its reference angle first. As $150^{\circ}$ terminates in quadrant II, The reference angle = $180^{\circ} - 150^{\circ}$ = $30^{\circ}$ As $150^{\circ}$ terminates in quadrant II, its cos will be negative. Therefore by reference angle theorem- $\cos 150^{\circ}$ = - $\cos30^{\circ}$ = - $\frac{\sqrt 3}{2}$