## Trigonometry (11th Edition) Clone

$-1$
Convert the angle measure to degrees to obtain: $=3\pi \cdot \frac{180^o}{\pi} = 540^o$ Thus, $\cos{3\pi} = \cos{540^o}$ 540 degrees is on the negative x-axis and is coterminal with $540^o-360^o=180^o$. From Section 1.3 (page 25) , we learned that: $\cos{180^o} = -1$ This means that: $\cos{540^o} = \cos{180^o} = -1$