Practice Problem (Checkpoint 1.18) Evaluate $\cos \frac{3\pi}{4}$ and $\sin (-\frac{\pi}{6})$. (Draw a diagram to illustrate your solution.)
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The angle $\frac{3\pi}{4}$ is in the second quadrant. The reference angle is $\pi - \frac{3\pi}{4} = \frac{\pi}{4}$. Since cosine is negative in the second quadrant, $\cos \frac{3\pi}{4} = -\cos \frac{\pi}{4} = -\frac{\sqrt{2}}{2}$. Show more…
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