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