Angles are a fundamental concept in geometry and trigonometry. Before we can understand that notion, we must recall the definition of a ray in the plane; this is a building block of the idea of an angle, so it is appropriate to begin there.
A ray in the plane is equivalent to a half-line in the plane. There is a starting point, or initial point, to the ray, and the points on the ray extend in a straight-line path from that initial point endlessly in one direction. We describe a ray by specifying its initial point A, a second point on the ray B, and using the notation . The point B is NOT the terminal point of the ray, because a ray has no terminal point.
An angle is the amount of rotation between two intersecting lines or rays in a plane. Angles are typically measured in degrees (°) or radians (rad). When considered as the amount of rotation between two rays, those rays are positioned in such a way that their initial points coincide. When looked upon as the amount of rotation between two intersecting lines, the point of intersection is viewed as the initial point, and the half-lines created by breaking the lines at this point form four rays between which angles can be identified.
Note that if intersecting lines are viewed as forming ...
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. The point B is NOT the terminal point of the ray, because a ray has no terminal point.