Mathematically, the minimum positive angle formed by the intersection of two straight lines (or vectors) is called the included angle between the two straight lines (or vectors), which is usually recorded as ∠ θ (included angle), and the interval range of the included angle is {θ| 0 ≤θ≤π}.

Angle is widely used in geometry and trigonometry.

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

1, zero degree angle

The angle is equal to 0, or a line.

2. acute angle

Angle greater than 0 and less than 90, or radian greater than 0 and less than {\displaystyle \pi /2}.

Step 3: Right angle

The angle is equal to 90, or the angle with radian of {\displaystyle \pi /2}.

4, obtuse angle

The angle is greater than 90 and less than 180, or the radian is greater than {\displaystyle \pi /2} and less than {\displaystyle \pi}.

5. Boxer angle

The angle is equal to 180, or the angle with radian of {\displaystyle \pi}.

6. Dominant angle or dihedral angle

The angle is greater than 180 and less than 360, or the radian is greater than {\displaystyle \pi} and less than {\displaystyle 2\pi}.

7. round angle

The angle is equal to 360, or the angle with radian of {\displaystyle 2\pi}.

Refer to Baidu Encyclopedia.