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Year 6

Explain how to calculate the area of a triangle

I can explain how to calculate the area of a triangle.

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New
New
Year 6

Explain how to calculate the area of a triangle

I can explain how to calculate the area of a triangle.

Download all resources
Share activities with pupils

Lesson details

Key learning points

  1. The area of a triangle is half the area of a parallelogram with same base and perpendicular height.
  2. The area of a triangle can be calculated using the formula area = base × perpendicular height ÷ 2

Common misconception

Pupils often over-generalise the word 'base' and do not recognise that the base is not always horizontal or the 'bottom' part of the shape.

Take time to identify that the base is related to the perpendicular height and show that one triangle can have a different base dependent on where the measurement is taken from. Pupils should consider efficiency when selecting a base.

Keywords

  • Parallelogram - A parallelogram is a quadrilateral with two pairs of parallel and equal sides.

  • Base - The base is the side which is perpendicular to the shape’s height.

  • Perpendicular - Two lines are perpendicular if they meet at a right angle.

Show triangles where the base is identified but not horizontal and model rotating the triangle until the base is horizontal, then reposition again. Reinforce that it was still the base regardless of its orientation.
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).

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Starter quiz

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6 Questions

Q1.
What is half of 36?
Correct Answer: 18
Q2.
What does congruent mean?
Correct answer: Exactly the same size and shape.
Completely different in all ways.
Exactly the same shape but smaller or bigger.
Q3.
How many of the side lengths are the same in a scalene triangle?
Correct Answer: 0, none, zero
Q4.
How do you calculate the area of a rectangle?
Correct answer: Base × perpendicular height
Base + perpendicular height
Correct answer: Length × width
Add the side lengths together.
Q5.
How do you calculate the area of a parallelogram?
Correct answer: Base × perpendicular height
Base + perpendicular height
Length × width
Add the side lengths together.
Q6.
15 × 14 =
Correct Answer: 210

Exit quiz

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6 Questions

Q1.
Which of the dotted lines shows the perpendicular height of the triangle?
An image in a quiz
Correct answer: Line a
Line b
Correct answer: Line c
Correct answer: Line d
Q2.
How do you calculate the area of a triangle?
Multiply the base by the perpendicular height.
Correct answer: Multiply the base by the perpendicular height and halve it.
Multiply the base by the perpendicular height and double it.
Q3.
Which of these show the correct formula for calculating the area of a triangle?
Area = base + perpendicular height × 2
Correct answer: Area = base × perpendicular height ÷ 2
Correct answer: Area = perpendicular height × base ÷ 2
Area = base ÷ perpendicular height × 2
Q4.
Calculate the area of the triangle. cm².
An image in a quiz
Correct Answer: 10
Q5.
Calculate the area of the triangle. cm².
An image in a quiz
Correct Answer: 42
Q6.
Calculate the area of a triangle with a base of 14 cm and a perpendicular height of 9 cm. cm².
Correct Answer: 63