Evaluate cos 210° without using a calculator by using ratios in a reference triangle.

210 degrees is a third quadrant degree angle that is in between 180 degrees and 270 degrees. So we expect that the cosine of this angle is equal to a negative value. In this case, the terminal angle of 210 degrees is 30 degrees. Hence, negative cosine  of 30 deg is equal to -sqrt 3/2

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