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Lesson 3 of 12

Finding the perimeter of polygons

I can use the properties of a range of polygons to deduce their perimeters, including generalising a perimeter formula where appropriate.

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Lesson 3 of 12
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Finding the perimeter of polygons

I can use the properties of a range of polygons to deduce their perimeters, including generalising a perimeter formula where appropriate.

Lesson details

Key learning points

  1. You can identify how to find the perimeter of polygons.
  2. The perimeter of any polygon can be found by adding but this may be time consuming.
  3. You can use a formula to find the perimeter of rectangle.
  4. You can find and use a formula to find the perimeter of any regular polygon.

Keywords

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

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

Common misconception

Pupils may not count all sides when calculating the perimeter due to the complexity of the shape.

Encourage pupils to mark the sides as they record the measurement to ensure they count every side length.

Pupils may feel that one method is good enough and do not need to know alternative methods. Evaluating and critiquing their own work is a valuable skill so they should be encouraged to do this.
Teacher tip

Equipment

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

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6 Questions

Q1.
Perimeter is the distance a 2D shape

Correct Answer: around

Q2.
Replacing a letter in an expression with a value is known as ______

formalisation
equalisation
subtraction
Correct answer: substitution

Q3.
If $$a$$ = 10 and $$b$$ = 7, what is the value of the expression 2$$b$$ + 2$$a$$?

Correct Answer: 34, Thirty four, thirty-four, thirty four

Q4.
Substitute the value $$y$$ = 2 into the expression 3$$y$$ $$+$$ 5 and evaluate the result

Correct Answer: 11, eleven, Eleven

Q5.
Izzy wants to add these values: 0.24, 0.5 and 17.3 together using column addition. Select the correct layout.

An image in a quiz
Values right aligned
Values left aligned
Values centered
Correct answer: Values aligned by the decimal point

Q6.
Evaluate 18.41 $$+$$ 16.7

3.448
3.511
34.48
Correct answer: 35.11

Assessment exit quiz

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6 Questions

Q1.
Perimeter is the distance around a 2D shape. It can be calculated by all of the edges together.

Correct Answer: summing, adding

Q2.
True or false? Number bonds can be used to calculate the perimeter more efficiently.

Correct answer: True
False

Q3.
Which of these formulas could not be used to calculate the perimeter ($$P$$) of this square?

An image in a quiz
$$P = 4b$$
$$P = 2b + 2b$$
$$P = 3b + b$$
Correct answer: $$P = b + b + b + b + b$$

Q4.
Complete the formula for the perimeter of this trapezium. $$P = $$

An image in a quiz
Correct Answer: a + b + 2d, a + b + d + d

Q5.
You can write a to calculate the perimeter in an efficient way.

Correct Answer: formula

Q6.
What is the mathematical name for the polygon whose perimeter ($$P$$) can be calculated using the formula $$P = t + r + 2w + y$$?

triangle
quadrilateral
Correct answer: pentagon
hexagon

Lesson appears in

UnitMaths / Perimeter and area

Maths

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