Topics
10. Shapes and angles
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Australia
Year 5

10.06 Measure and compare angles

Lesson
Practice

Are you ready?

Let's review how to identify angles.

Examples

Example 1

Which of the following is a right angle?

A
A right angle
B
An acute angle.
Worked Solution
Create a strategy

Look for the angle with a small square in the corner.

Apply the idea

Option A has a small square at the corner, so the answer is option A.

Idea summary

A right angle is exactly 90\degree. It can also be called a quarter turn.

Compare angles

Let's see how we can compare two angles of a skateboard ramp.

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Examples

Example 2

Which angle is smaller?

A
A protractor with an angle of 95 degrees drawn on it. Ask your teacher for more information.
B
A protractor with an angle of 81 degrees drawn on it. Ask your teacher for more information.
Worked Solution
Create a strategy

Look at the numbers that the lines go through.

Apply the idea

In option A, the horizontal line goes through the 0 on the outside semi circle. So to read the angle, we look at the outside number on the other line of the angle which is at approximately 95 \degree .

In option B, the horizontal line goes through the 0 on the inside semi circle. So to read the angle, we look at the inside number on the other line of the angle which is at approximately 80 \degree .

We can also see that option B has a smaller angle just by looking how large the angle turn referencing from 0\degree.

So the correct answer is option B.

Idea summary

Angles can be compared by how large the turn is between the lines.

Measure angles with a protractor

We can measure the exact angle, using a special tool called a protractor. Let's see how to use one.

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Examples

Example 3

What is the angle being measured by this protractor?

This image shows a protractor with an angle of 160 degrees. Ask your teacher for more information.
Worked Solution
Create a strategy

Look at the numbers that the lines go through.

Apply the idea

The horizontal line goes through the numbers 0 on the inside, so we should use the inside number on the other line to measure the angle.

The other line goes through 160 on the inside, so the size of the angle is 160 \degree.

Idea summary

A protractor is an instrument that allows us to measure an angle. Start with one arm of the angle on the 0\degree line and read the amount of turn required to reach the second arm.

An image of a protractor.

Outcomes

AC9M5M04

estimate, construct and measure angles in degrees, using appropriate tools including a protractor, and relate these measures to angle names

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