Primary 5 Area of Triangle
Keywords
- Base
- The side chosen to measure the triangle's height from.
- Perpendicular height
- The shortest distance from the base line to the opposite corner, meeting the base at a right angle.
- Right angle
- A 90° angle. Look for it where the height meets the base line.
- Area
- The space inside a shape, measured in square units such as cm².
Base and height
Choose a side as the base. The height is always perpendicular to that base, from the opposite corner to the base line.
Choose a triangle
Choose a height to see its matching base.
See worked example
In the acute triangle, QR is the 6 cm base. Which height matches it?
Choose the height from P. It meets QR at a right angle and measures 4 cm.
Answer: Height from P, 4 cm.
Try together
In the obtuse triangle, why is one height outside and the other inside?
Reveal the answer
Each height belongs to a different chosen base. Both meet their own base line at a right angle.
Area of Triangle
Start with a rectangle. Draw a diagonal to split it into two equal triangles. One triangle has half the rectangle's area.
Why do we take half?
Area of triangle = 12 × base × perpendicular height
Base = 6 cm; perpendicular height = 4 cm. Why is its area 12 cm²?
See worked example
A right triangle has a base of 8 cm and a perpendicular height of 3 cm. Find its area.
- Base × height = 8 × 3 = 24 cm².
- Area = 24 ÷ 2 = 12 cm².
Answer: 12 cm².
Try together
An obtuse triangle has a base of 10 cm and a perpendicular height of 5 cm outside the shape. Find its area.
Reveal the answer
Area = 10 × 5 ÷ 2 = 25 cm². The same formula works when the height is outside.