Know that unknown side lengths can be calculated from the perimeter and known side lengths | Maths

Starter quiz

Complete the sentence. Half of 36 is ______
- '18' ✓
Which calculations will give the perimeter of this rectangle?
- 8 + 4
- 8 + 4, then double the answer ✓
- Double 8 + double 4 ✓
- It is impossible to give the perimeter because there are missing side lengths.
What is the perimeter of this rectangle?
- 11 units
- 16 units
- 22 units ✓
What is the perimeter of this rectangle? ______ units.
- '24' ✓
What is the perimeter of this rectangle? ______ units.
- '180' ✓
Order these rectangles from shortest to longest perimeters.
- 1
  
  ⇔
  
  Rectangle C with sides of 4 and 5 units
- 2
  
  ⇔
  
  Rectangle A with sides of 2 and 8 units
- 3
  
  ⇔
  
  Rectangle B with sides of 6 and 5 units

Exit quiz

The rectangle has a perimeter of 30 units. What is the length of each missing side? ______ units.
- '5' ✓
The rectangle has a perimeter of 20 units. What is the length of each missing side? ______ units.
- '7' ✓
This rectangle has a perimeter of 22 units. The long side of this shape is given. What is the length of each short side? ______ units.
- '5' ✓
A rectangle has a perimeter of 30 units. The long side has a length of 13 units. What is the length of each short side? ______ units.
- '2' ✓
A rectangle has a perimeter of 30 units. The short side has a length of 1 unit. What is the length of each long side? ______ units.
- '14' ✓
Which of these are possible side lengths for a rectangle with a perimeter of 50 cm?
- 1 cm, 1 cm, 24 cm , 24 cm ✓
- 9 cm, 11 cm, 19 cm, 11 cm
- 10 cm, 10 cm , 10 cm , 10 cm
- 15 cm, 15 cm, 20 cm, 20 cm
- 12 cm, 12 cm, 13 cm, 13 cm ✓

Worksheet

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Presentation

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Lesson Details

Key learning points

Subtraction and halving can be used to calculate the missing side length of a rectangle.
If you know the perimeter and one of the side lengths you can subtract the known sides and halve the difference.
Mental and written methods can be used to calculate the perimeter of a 2D shape.

Common misconception

Pupils may see a rectangle with the perimeter given and only one of the side lengths and think that there is insufficient information as there are three missing side lengths.

Remind pupils that because it is a rectangle, the side length opposite the given side length is in fact known. A counter can be added on and then removed again to show that they actually know two of the missing side lengths, not just one.

Keywords

Perimeter - The distance around a two-dimensional shape.