Topics
7. Measurement
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AustraliaNSW
Stage 3

7.03 Perimeter of rectangles

Lesson
Practice
Lesson

Are you ready?

If we can add three or more numbers together, that can help us in this lesson. Let's try this problem to practice.

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 of rectangles

This video looks at using the properties of rectangles to calculate the perimeter, it also talks about some special notation that is often used to show when two sides have the same length.

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Examples

Example 2

Find the perimeter of the rectangle shown.

The image shows a rectangle with width of 9 metres and length of 23 metres.
Worked Solution
Create a strategy

We can use the formula for a rectangle: \text{Perimeter}=2 \times (\text{Length} + \text{Width} ).

Apply the idea

First add the length and width:

\displaystyle \text{Length$+$Width}\displaystyle =\displaystyle 23+9Add the values
\displaystyle =\displaystyle 32 \text{ m}

Now multiply this by 2 to get the perimeter:

\displaystyle \text{Perimeter}\displaystyle =\displaystyle 2 \times (\text{Length} + \text{Width} )Use the formula
\displaystyle =\displaystyle 2 \times 32Double 32
\displaystyle =\displaystyle 64 \text{ m}
Idea summary

Because a rectangle has 2 pairs of sides with equal length, we can add the length and width, then double it, to find the perimeter of a rectangle.

A square is a special kind of rectangle, with 4 sides the same length, so we can multiply its side length by 4.

Outcomes

MA3-9MG

selects and uses the appropriate unit and device to measure lengths and distances, calculates perimeters, and converts between units of length

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