New
New
Explain how to calculate the area of a triangle
I can explain how to calculate the area of a triangle.
Lesson 3 of 10
New
New
- Year 6
Explain how to calculate the area of a triangle
I can explain how to calculate the area of a triangle.
Lesson details
Key learning points
- The area of a triangle is half the area of a parallelogram with same base and perpendicular height.
- The area of a triangle can be calculated using the formula area = base × perpendicular height ÷ 2
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.
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.
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 (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.What is half of 36?
What is half of 36?
Q2.What does congruent mean?
What does congruent mean?
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?
How many of the side lengths are the same in a scalene triangle?
Q4.How do you calculate the area of a rectangle?
How do you calculate the area of a rectangle?
Base + perpendicular height
Add the side lengths together.
Q5.How do you calculate the area of a parallelogram?
How do you calculate the area of a parallelogram?
Base + perpendicular height
Length × width
Add the side lengths together.
Q6.15 × 14 =
15 × 14 =
Assessment exit quiz
6 Questions
Q1.Which of the dotted lines shows the perpendicular height of the triangle?
Which of the dotted lines shows the perpendicular height of the triangle?
Line b
Q2.How do you calculate the area of a triangle?
How do you calculate the area of a triangle?
Multiply the base by the perpendicular height.
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?
Which of these show the correct formula for calculating the area of a triangle?
Area = base + perpendicular height × 2
Area = base ÷ perpendicular height × 2
Q4.Calculate the area of the triangle. cm².
Calculate the area of the triangle. cm².
Q5.Calculate the area of the triangle. cm².
Calculate the area of the triangle. cm².
Q6.Calculate the area of a triangle with a base of 14 cm and a perpendicular height of 9 cm. cm².
Calculate the area of a triangle with a base of 14 cm and a perpendicular height of 9 cm. cm².
