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

I can calculate unknown side lengths of rectangles from the perimeter and known side lengths.

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Year 4

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

I can calculate unknown side lengths of rectangles from the perimeter and known side lengths.

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Share activities with pupils

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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.

Cycle A uses two-coloured counters to as a means exploring ways to find missing side lengths when the perimeter is known. The Practice tasks are not presented with counters, but including them could be a useful way of scaffolding learning for pupils who are finding the concept challenging.

Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).