1. Drawing: In the plane rectangular coordinate system, the points are classified by quadrants, and then the boundary line of each quadrant is drawn, so that the concept of quadrants can be understood more intuitively.
2. Expressed by angle: divide the angle into positive angle, negative angle and zero angle, and then correspond to the first, second and third quadrants respectively.
3. Represented by trigonometric function: sine, cosine and tangent correspond to the first, second and third quadrants respectively.