Find the exact value of the expression.
-$\cos 210 ^ { \circ }$
A) $- \frac { \sqrt { 2 } } { 2 }$
B) $\frac { \sqrt { 2 } } { 2 }$
C) $\frac { \sqrt { 3 } } { 2 }$
D) $- \frac { \sqrt { 3 } } { 2 }$

Question

Quizplus · Accepted Answer

The angle 210° is in the third quadrant where cosine is negative. The reference angle for 210° is 210° - 180° = 30°. The cosine of 30° is $\sqrt{3}/2$, so $\cos 210° = -\sqrt{3}/2$.

Find the Exact Value of the Expression cos⁡210∘\cos 210 ^ { \circ }cos210∘

Find the Exact Value of the Expression $\cos 210 ^ { \circ }$