Which of the following best explains why cosine 2 pi over 3 is not equal to cosine 5 pi over 3 ?

The angle 2π/3 is in Quadrant II, where cosine is negative, so cos(2π/3) = −1/2. The angle 5π/3 is in Quadrant IV, where cosine is positive, so cos(5π/3) = 1/2. Therefore, choice B is correct: cosine is negative in the second quadrant and positive in the fourth quadrant.

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