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On a Unit Circle, the cosine is the x coordinate of the point on the circle represented by an angle. Angles greater than 90° (pi/2 radians) and less than 270° (3*pi/2 radians) are to the left of the y-axis, so x is negative.

Quadrant I is the upper right quadrant (x positive, y positive) 0° < ɵ < 90°

Quadrant II is the upper left quadrant (x negative, y positive) 90° < ɵ < 180°

Quadrant III is the lower left quadrant (x negative, y negative) 180° < ɵ < 270°

Quadrant IV is the lower right quadrant (x positive, y negative) 270° < ɵ < 360°

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Q: Why is cosine negative in the third quadrant?
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