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Year 7

Calculating missing lengths with perimeter of polygons

You can use the properties of a range of polygons and the perimeter of a given polygon to deduce missing side lengths.

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Year 7

Calculating missing lengths with perimeter of polygons

You can use the properties of a range of polygons and the perimeter of a given polygon to deduce missing side lengths.

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Share activities with pupils
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Lesson details

Key learning points

  1. Missing side lengths can be found if the perimeter is known.
  2. Perimeter problems involve a context where the distance around a 2D shape is wanted.

Keywords

  • Perimeter - The perimeter is the distance around a 2D shape.

  • Polygon - A polygon is a flat (2D), closed figure made up of straight line segments.

Common misconception

Pupils may struggle to think of a different context to apply to finding the perimeter.

An alternative context could be: putting a ribbon around a cake and leaving space for decoration.

It can be beneficial to have a list of alternative contexts available should pupils need additional support. A football pitch with the outer border marked and space left for the goals would also work here.
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).

Lesson video

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Starter quiz

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6 Questions

Q1.
A polygon is a
Correct answer: flat (2D), closed figure made up of straight line segments
flat (2D), closed figure made up of curved line segments
3D object made up of straight line segments
flat (2D), open figure made up of straight line segments
Q2.
True or false? All parallelograms are rhombuses.
True
Correct answer: False
Q3.
Calculate the perimeter of the square.
An image in a quiz
Correct Answer: 28 cm, 28 centimeters, twenty-eight cm, twenty-eight centimeters, twenty eight cm
Q4.
Calculate the perimeter of the rectangle.
An image in a quiz
Correct Answer: 16 cm, 16 centimeters, sixteen cm, sixteen centimeters, sixteen centimetres
Q5.
Calculate the perimeter of the kite.
An image in a quiz
7.3 cm
9.3 cm
10.6 cm
Correct answer: 14.6 cm
Q6.
An irregular polygon has the following side lengths: 5 cm, 8 cm, 6 cm, 7 cm, and 9 cm. What is the perimeter of the polygon?
Correct Answer: 35 cm, 35 centimeters, thirty-five centimeters, thirty five centimeters

Exit quiz

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6 Questions

Q1.
True of false? Summing the length and width of a rectangle gives a result that is half of the perimeter of the rectangle.
Correct answer: True
False
Q2.
The perimeter of the octagon is 184 cm. What is the length of one side?
An image in a quiz
Correct Answer: 23 cm, 23 centimeters, twenty-three centimeters, twenty three centimeters
Q3.
Two line segments are congruent if they are ______
parallel
perpendicular
Correct answer: the same length
different lengths
Q4.
A rectangular garden has a length of 12 meters and a width of 8 meters. What is the perimeter of the garden?
20 m
Correct answer: 40 m
60 m
96 m
Q5.
Lucas is planning to build a rectangular flowerbed in the backyard. They want the flowerbed to have a perimeter of 36 m. If the width of the flowerbed is 6 m, what should be the flowerbed length?
6 m
Correct answer: 12 m
15 m
18 m
Q6.
Dany has a triangular room. The room has side lengths measuring 7 m, 9 m, and $$x$$ m. The perimeter of the triangular room is 28 m. What is the length of side $$x$$?
Correct Answer: 12 m, 12 meters, twelve meters, twelve m