## Trigonometry (11th Edition) Clone

$-\frac{1}{2}$
RECALL: $\sin{s} = y \\\cos{s} = x \\\tan{s} = \frac{y}{x} \\\cot{s} = \frac{x}{y} \\\sec{s} = \frac{1}{x} \\\csc{s}=\frac{1}{y}$ (refer to Figure 11 on page 111 of the textbook) A negative angle measure means the terminal side of the angle will move clockwise from the positive x-axis. Thus, from the positive x-axis, moving the terminal side $\frac{4\pi}{3}$ radians clockwise ends at $\frac{2\pi}{3}$. The angle $\frac{2\pi}{3}$ intersects the unit circle at the point $(-\frac{1}{2}, \frac{\sqrt3}{2})$. This point has: $x= -\frac{1}{2}$ $y=\frac{\sqrt3}{2}$ Thus, $\cos{(-\frac{4\pi}{3}} \\= \cos{\frac{2\pi}{3}} \\=-\frac{1}{2}$