Answer
$-1$
Work Step by Step
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:
$\cos{540^o} = \cos{180^o} = -1$
