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Calculating missing lengths with perimeter of polygons

I 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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Lesson 4 of 12

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Year 7

Calculating missing lengths with perimeter of polygons

I 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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Lesson details

Key learning points

Missing side lengths can be found if the perimeter is known.
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

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Licence

This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Worksheet

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Prior knowledge 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.

Correct Answer: 28 cm, 28 centimeters, twenty-eight cm, twenty-eight centimeters, twenty eight cm

Q4.
Calculate the perimeter of the rectangle.

Correct Answer: 16 cm, 16 centimeters, sixteen cm, sixteen centimeters, sixteen centimetres

Q5.
Calculate the perimeter of the kite.

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

Assessment 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?

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

Lesson appears in

UnitMaths / Perimeter and area

Maths

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Calculating missing lengths with perimeter of polygons

Calculating missing lengths with perimeter of polygons

Lesson slides

Lesson details

Key learning points

Keywords

Common misconception

Equipment

Licence

Lesson video

Worksheet

Prior knowledge starter quiz

6 Questions

Q1.A polygon is a

Q2.True or false? All parallelograms are rhombuses.

Q3.Calculate the perimeter of the square.

Q4.Calculate the perimeter of the rectangle.

Q5.Calculate the perimeter of the kite.

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?

Assessment exit quiz

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.

Q2.The perimeter of the octagon is 184 cm. What is the length of one side?

Q3.Two line segments are congruent if they are ______

Q4.A rectangular garden has a length of 12 meters and a width of 8 meters. What is the perimeter of the garden?

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?

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 $$x$$?

Lesson appears in

UnitMaths / Perimeter and area

Maths

Q1.
A polygon is a

Q2.
True or false? All parallelograms are rhombuses.

Q3.
Calculate the perimeter of the square.

Q4.
Calculate the perimeter of the rectangle.

Q5.
Calculate the perimeter of the kite.

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?

Q1.
True of false? Summing the length and width of a rectangle gives a result that is half of the perimeter of the rectangle.

Q2.
The perimeter of the octagon is 184 cm. What is the length of one side?

Q3.
Two line segments are congruent if they are ______

Q4.
A rectangular garden has a length of 12 meters and a width of 8 meters. What is the perimeter of the garden?

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?

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 $$x$$?