Find the exact circular function value.
-$$\cos \frac { - 2 \pi } { 3 }$$
A) undefined
B) $- \frac { 1 } { 2 }$
C) $- \frac { \sqrt { 3 } } { 2 }$
D) $\frac { \sqrt { 3 } } { 2 }$

Question

Quizplus · Accepted Answer

The cosine function is periodic with a period of $2\pi$, so $\cos \frac{-2\pi}{3}$ is equivalent to $\cos \frac{4\pi}{3}$ (since adding $2\pi$ does not change the value). At $\frac{4\pi}{3}$, the cosine function has a value of $-\frac{1}{2}$, which corresponds to the x-coordinate of the point on the unit circle at that angle.

Find the Exact Circular Function Value cos⁡−2π3\cos \frac { - 2 \pi } { 3 }cos3−2π​

Find the Exact Circular Function Value $\cos \frac { - 2 \pi } { 3 }$