## The Lesson

An angle is created by two rays that have a common end point, called the vertex. The angle is also a measure of the rotation between the two rays. Angles can be measured in degrees (°) or in radians.

## How to Identify an Angle

Angles are often identified using Greek letters (α, β, γ, θ, ψ). The letter theta θ is often used. In geometric figures, points are often labelled with capital letters. Consider the angle made between points ABC, where the vertex is A and AB and AC are the rays: The angle can be identified as the angle A, or by ∠BAC, where the middle of the 3 letters is the vertex.

## How to Measure an Angle

An angle is measured by the shortest rotation between the two rays. Angles are often measure in degrees, denoted °. There are 360 degrees in a full rotation. Angles can also be measured in radians. There are 2π radians in a full rotation. Angles can be measured directly using a protractor: Angles can also be calculated using geometry and trigonometry.

## Types of Angles

The different types of angles are:
Type of Angle Description Acute Less than 90° Right 90° Obtuse Greater than 90° but less than 180° Straight 180° Reflex Greater than 180° Full 360°

## Interactive Game on the Types of Angles  ## Parts of an Angle An angle is created by two rays meeting at a common endpoint, the vertex.
• A ray is a line which starts from a point.
• A vertex is the point where the two rays meet.

## What's in a Name?

Angle comes from the Latin word "angulus" meaning "corner". It has the same root as the word "ankle", because the foot forms a corner with the leg. 