In this activity, we will learn how to estimate the size of each angle.

Remember:

Acute angles are smaller than 90°

Right angles are exactly 90°

Obtuse angles are larger than 90°

Reflex angles are larger than 180°

Let's try an example.

Example

Estimate the size of this angle.

Think of key angles such as: 30°, 60°, 90°, 120° and 180°

A right angle is 90°, so 30° is a third of a right angle and 60° is two thirds of a right angle.

A straight line is 180°, so 120° is a right angle plus a third of a right angle.

Answer

The angle is about one third more than a right angle

The angle is about 120°

Using a protractor, we would find that the exact size of this angle = 116°

Are you ready to have a go at some of these yourself now?