In Exercises 5–18, the unit circle at the top of the next column has been divided into twelve equal arcs, corresponding to t-values of $0, \frac{\pi}{6}, \frac{\pi}{3}, \frac{\pi}{2}, \frac{2 \pi}{3}, \frac{5 \pi}{6}, \pi, \frac{7 \pi}{6}, \frac{4 \pi}{3}, \frac{3 \pi}{2}, \frac{5 \pi}{3}, \frac{11 \pi}{6},$ and 2$\pi$ Use the (x, y) coordinates in the fi gure to fi nd the value of each trigonometric function at the indicated real number, t, or state that the expression is undefined.
$$
\cos \frac{2 \pi}{3}
$$