Warm-up
Three quick questions from earlier lessons.
Discover
Two copies of the same triangle fit together into a parallelogram with base 6 and height 4. What is the area of one triangle?
- 12
- 24
Slide C along the dashed line. What happens to the area?
- It stays 12
- It changes with the slant of the sides
Key idea
A triangle is half a parallelogram.
Area of a triangle , with the height to the chosen base (measured at a right angle; it may fall outside the triangle).
Worked example
Base 10 cm, height 7 cm.
cm²
Half of base × height.
Your turn, guided
Your turn: base 9, height 6. The area.
½ · 54.
Practice ★
- A triangle has a base of 15 cm and a height of 4 cm. What is its area (cm²)?
- A triangle has a base of 11 cm and a height of 4 cm. What is its area (cm²)?
- A triangle has a base of 7 m, a height of 6 m and another side of 11 m. What is its area (m²)?
Watch out!
Practice ★★
- A triangle has an area of 32 cm² and a height of 8 cm. What is its base (cm)?
- A triangle has an area of 40 cm² and a height of 4 cm. What is its base (cm)?
- Boss problem. A tent is a prism. Its front is a triangle with base 8 ft and height 4 ft, and the tent is 10 ft long.Not to scale
Exit ticket
Three questions to check you have got it.
- A triangle has a base of 15 cm and a height of 4 cm. What is its area (cm²)?
- A triangle has a base of 11 cm and a height of 4 cm. What is its area (cm²)?
- A triangle has a base of 20 cm and a height of 9 cm. What is its area (cm²)?
Summary card
Area of triangles
.
Watch out: do not forget the ½.