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      Calculating missing lengths with perimeter of polygons

      Lesson details

      Learning outcome

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

      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.

      Teacher tip

      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.

      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).

      Lesson video

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      Prior knowledge starter quiz

      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

      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

      To help you plan your 7 maths lesson on: Calculating missing lengths with perimeter of polygons, download all teaching resources for free and adapt to suit your pupils' needs...