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Australia
Year 5

7.02 Perimeter

Lesson

Are you ready?

Can you  add multiple numbers  together?

Examples

Example 1

Solve 40 + 20 + 3.

Worked Solution
Create a strategy

Use a number line.

Apply the idea

Plot the first number on the number line:

3040506070

Then jump forward by 20 to get 60:

3040506070

Then jump forward by 3 to get 63:

3040506070

40+20+3=63

Idea summary

When adding numbers we can use a number line or add digits by place value.

Perimeter using unit squares

The perimeter of a shape is the length around the outside of the shape.

If a shape is made up of unit squares, we can use those to count the distance around the outside of the whole shape. We count the outside edges of every unit square, making sure we include the whole outside of our shape.

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Examples

Example 2

Find the perimeter of the shaded region shown, where each square has a length of 1 unit.

The shaded region has 6 sides and is made up of 5 square tiles. Ask your teacher for more information.
Worked Solution
Create a strategy

We can count the unit edges outside the shaded shape.

Apply the idea

The shape has 10 unit edges around it.

Perimeter =10 units

Idea summary

To find the perimeter we count the edge of every unit square, around the outside of our shape.

Perimeter

2D shapes are also known as polygons, and we've seen some of these shapes before. This video looks at how to work out the perimeter of any polygon.

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Examples

Example 3

Find the perimeter of the figure shown.

Irregular polygon with different side lengths. Ask your teacher for more information.
Worked Solution
Create a strategy

Add all the lengths of the sides to find the perimeter.

Apply the idea
\displaystyle \text{Perimeter}\displaystyle =\displaystyle 4 + 8 + 8 + 16 + 12 Add the lengths of the sides
\displaystyle =\displaystyle 48 \text{ m}
Idea summary

It doesn't matter what shape our polygon is, as long as we add the length of every side, we can work out the perimeter.

We must also write the unit of measurement with our answers, such as \text{cm, m, mm}, or \text{km}.

Outcomes

ACMMG109

Calculate perimeter and area of rectangles using familiar metric units

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