In a plane, a ray rotates around its endpoint in two opposite directions: clockwise and counterclockwise. Traditionally, the angle formed by counterclockwise rotation is called positive angle; The angle formed by clockwise rotation is called negative angle; When the light does not rotate, we also regard it as an angle, which is called a zero-degree angle. When the light rotates counterclockwise or clockwise around its endpoint, the absolute amount of rotation can be arbitrary. When drawing, arcs with arrows are often used to indicate the direction of rotation and the absolute amount of rotation. The angle generated by rotation is usually called the rotation angle.

After the above promotion, the concept of angle should include positive angle, negative angle and zero angle, that is, any angle can be formed.

Note: (1) "Angle α" or "∠ α" can be simplified to "α" without causing confusion; (2) If α is a zero-degree angle α = 0, the terminal edge of the zero-degree angle coincides with the initial edge; (3) The concept of angle has been extended to include positive angle, negative angle and zero angle. When discussing an angle in a rectangular coordinate system, the vertex of the angle coincides with the origin of the coordinate, the starting edge of the angle is on the non-negative semi-axis of the X axis, and the ending edge of the angle is in which quadrant, so we can say that the angle is this quadrant (or which quadrant it belongs to).

If the terminal edge of an angle is on the coordinate axis, it is considered that the angle is not in any quadrant.

Representation method of quadrant angle

The first quadrant k 360+0

The second quadrant k 360+90