The Concept of Angle in Trigonometry
In trigonometry, an angle is formed by two rays with the same initial point.
Let us study two rays: OK and OM, where OM is a moving ray that can rotate about point O.
Point O, relative to which the rotation takes place, is called the ‘center of rotation’ or a vertex.
OK is a fixed ray or the initial side of the angle;
ОМ is called a radius vector or the terminal side of the angle.
Determining the Angle of Rotation Using the Radius Vector