Perimeter
Maths: Year 5 & 6 National Curriculum, topic 2.2.1
Perimeter
The perimeter of a shape is the 'outside' of the shape. You need to add all of the lengths of the shape.
Example 1
- To find the perimeter of this shape, we need to add all the side lengths together.
Example 1 cont.
Example 2
- To find the perimeter of this shape, we need to add all the side lengths.
Example 2 cont.
Missing length
- Sometimes you might need to work out the missing length of a shape in order to find the perimeter.
- What is the perimeter of the rectangle above?
Missing length
- We use the information that rectangles have two pairs of sides of equal length to work out that the length of the missing side is 4 cm.
Perimeter of Composite Shapes
A composite shape can be made when two or more shapes are put together.
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Example
- This is a composite shape made up of two rectangles.
Perimeter of composite shapes
- To find the perimeter. you must first work out the length of the missing sides.
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Length of side a
- To work out the length of side a, you need to look at how long the length of the shape is altogether.
- You can see that the longest verticle side is 6 m.
- You need to match with the other verticle side, which is 3 m long.
- 3 m + ? = 6 m
- So, side a is 3m long.
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Length of side b
- To work out the length of side b, you need to look at how long the two horizontal lengths are.
- The horizontal lengths are 4 m and 5 m.
- 4 m + 5 m = 9 m
- So, side b is 9 m long.
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Calculating the perimeter
- You can now calculate the perimeter by adding all the lengths on the 'outside' of the shape.
Test yourself on Perimeter
What is the perimeter of the rectangle below?
What is the perimeter of this equilateral triangle?
What is the perimeter of this rectangle?
What is the perimeter of this composite shape?
What is the perimeter of this composite shape?