1. Summary of the concept of angle: In the plane geometry of junior high school, the figure composed of two rays drawn from a point is called angle, which is summarized as "a ray rotates around its endpoint to form an angle". Because the rotation direction is different, there are positive and negative angles. When the light does not rotate, it is also considered that an angle is formed at this time, and this angle is zero degree. In this way, the angle in trigonometric function is replaced by the moving point in plane geometry. Therefore, the angle discussed in trigonometric function can take any value, including an angle greater than 360 and an angle less than or equal to zero. (1) Rotating rays can form positive angle, negative angle and zero angle respectively. (2) quadrant angle: When learning trigonometric functions, we