Describe why the tangent ratio is positive only in quadrants 1 and 3

Between 0° and 90° [1st quadrant] tan Ф is always increasing, then positive:

tan (0°) = 0

tan (30°) = √ (3) / 3 = 0.577

tan (45°) = 1

tan (60°) = √3 = 1.732

tan (89°) = 57.28

We also know that tan (Ф) = tan (180° + Ф) [ 3rd quadrant]

The same logic as above applies

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