# The Concept of Angle in Trigonometry

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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