Euclid, the father of geometry, once defined an angle as the relative inclination of two non-parallel straight lines in a plane. Proclos thinks that angle may be a trait, a quantifiable quantity, or a relationship. Oldham thinks that an angle is a deviation from a straight line, and Cabus of Antioch thinks that an angle is a space between two intersecting straight lines. Euclid thinks that an angle is a relationship, but his definitions of right angle, acute angle or obtuse angle are quantitative.
Extended data:
Positive and negative angles
The above definition of angle does not take into account the case that the angle is negative. However, in some applications, the angle value will be symbolized to indicate that it rotates in different directions relative to the reference.
In the two-dimensional Cartesian coordinate system, the angle is generally based on the positive direction of the X axis. If it rotates in the positive direction of the Y axis, its angle is positive, and if it rotates in the negative direction of the Y axis, its angle is negative. If the two-dimensional Cartesian coordinate system is also the X-axis to the right and the Y-axis to the right, then the counterclockwise rotation corresponds to a positive angle and the clockwise rotation corresponds to a negative angle.
Generally speaking,? The angle θ is the same as the angle obtained by subtracting θ from a circle. Like what? 45 and 360? 45 (= 3 15) is equivalent, but it is only applicable to the case of relative position expressed by angle, not the concept of rotation. Spin? 45 is different from 3 15.