The included angle is usually expressed by the Greek letter ∠ θ, and its interval range is 0 ≤θ≤π/2. For the case of two vectors, the interval range of the included angle is 0 ≤θ≤π. The included angle is not only related to the inclination of straight lines, but also to their relative positions. In geometry, included angle is used to describe the angular relationship between two rays or line segments. In physics, the included angle is used to describe the angle between two vectors. These angles have clear physical significance. An included angle of zero indicates that the two vectors are in the same direction, and an included angle of 180 indicates that the two vectors are in opposite directions. The concept of included angle is widely used in mathematics, geometry and physics, which helps people to understand and describe the relative position and direction relationship between different objects.