GEOMETRY

Area Of Triangles

Learn the reliable method for area of triangles, then apply it to 11+ exam-style questions.

1 · WHAT YOU NEED TO KNOW

The idea

A triangle's area is half the area of a rectangle with the same base and perpendicular height.

Picture itCount the square units inside
Colourful playground markings showing shapes, equal sections, parallel lines and boundaries
2 · THE METHOD

Follow these steps

  1. Identify the base and perpendicular height.
  2. Multiply them and divide by two.
  3. Rearrange the same relationship if a length is missing.
rectangle area = length × width
3 · WORKED EXAMPLES

See the method in action

Example 1

The triangle's area and perpendicular height are shown. What is its base length?

area 30 cm²height 6 cm⊥
  1. From area = 1/2 × base × height, base = (2 × 30) ÷ 6 = 10 cm.
Answer: 10 cm
Example 2

The triangle's area and perpendicular height are shown. What is its base length?

area 30 cm²height 6 cm⊥
  1. From area = 1/2 × base × height, base = (2 × 30) ÷ 6 = 10 cm.
Answer: 10 cm
4 · ELEVENSPRINT TIP

A useful shortcut

Rearrange the same relationship if a length is missing.

5 · COMMON MISTAKES

Watch out for these

6 · TRY IT YOURSELF

Three fresh questions

Ready to check the method? You will get three fresh, validated questions.