Angles: types, notation and measuring
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Explainer Video
Summary
Angles: types, notation and measuring
In a nutshell
Angles can be categorised as acute, right-angled, obtuse or reflex. Angles can be measured with a protractor and they can be represented by three-letter notation.
Types of angles
| NAME | ANGLE RANGE | DIAGRAM |
| Acute | less than |  |
| Right angle | |  |
| Obtuse | between and |  |
| Reflex | between and |  |
Measuring angles with a protractor
An angle can be measured using a protractor. Line up the base line of the protractor with one of the lines forming the angle, with the cross of the protractor on top of the point of the angle. There are two scales on the protractor, one on the inside circle and one on the outside. Make sure to read the correct scale by finding where lines up with one of the lines forming the angle.
Example 1
Find the size of the angle shown.
Position the protractor on the angle and read the inside circle.
The angle is .
Three-letter angle notation
Three-letter angle notation is used when there are multiple angles on a diagram. The angles are written in the form , where the angle is at , and the lines forming the angle end at points and .
Example 2
Describe the right angle below in three-letter notation.
The right angle is at and the lines forming the angle go to points and .
Therefore, the right angle is .
FAQs - Frequently Asked Questions
What is three letter notation when describing angles?
Angles can be represented using three letter notation. The middle letter is where the angle lies, e.g. for angle ABC, the angle is at point B and the lines forming the angle end at points A and C.
How can angles be measured?
Angles can be measured with a protractor.
What are the different types of angles?
The different types of angles are acute angles, right angles, obtuse angles or reflex angles.