Make sure your calculator is in radian mode.

$\text{Since }\frac{4\pi}{3} \text{ is related to the }30\degree-60\degree-90\degree\text{ angles, this one}$

$\text{has an exact answer. Locate it on the unit circle.}$

$\text{Notice that it will be a reflection (twice) of sin} \left(\frac{\pi}{3}\right).$

It is in the third quadrant.

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