Idea 12of 22Builds onThe height goes straight up
Two triangles make a parallelogram
How is a triangle's area linked to a parallelogram's?
Today you'll discover
- how two copies fit together
- why a triangle is half
- the rule for any triangle
Think firstA triangle: base 6 cm, height 4 cm
Fit a copy, turned half a turn, against one side. What do the two make?
Decide first, then press Next. Watch the blue copy turn into place.
A triangle on a grid, base 6 cm in orange and a dashed blue height of 4 cm. A blue copy turns half a turn and fits against the right-hand side, making a parallelogram: 6 × 4 = 24 cm². The first triangle is half: 6 × 4 ÷ 2 = 12 cm². A right triangle, base 4 cm and height 3 cm, does the same: 6 cm². The last step shows a triangle with base 8 cm and height 5 cm.
See the whole ideathe big idea and the key word, in one place
The big idea
Two copies of a triangle make a parallelogram, so a triangle's area is base × height ÷ 2.
Pair, then half
Base × height gives the parallelogram. Halve it for one triangle.
- base6 cm
- height4 cm
- two copies24 cm²
- one triangle12 cm²
Easy to get wrongDon't forget the ÷ 2. 6 × 4 = 24 is the pair of triangles, not one.
Key word
- formula
- a rule for working something out, written with words or signs, like base × height ÷ 2
Ready to use it?
- Find the area of a triangle with base × height ÷ 2.