Idea 20of 24Builds onBuild a triangle from three sides
Two sides must beat the third
Can any three lengths make a triangle?
Today you'll discover
- what happens when two sides are too short
- a quick test for three lengths
- how to use it before building
Think firstAn 8 cm base; sides of 3 cm and 4 cm
Can sides of 3 cm and 4 cm meet above an 8 cm base?
Decide first, then press Next. Watch whether the two curves ever meet.
An 8 cm base from A to B. A 3 cm arc is swung from A and a 4 cm arc from B; they never meet, and the 1 cm gap between them along the base is marked: 3 cm + 4 cm = 7 cm, shorter than 8 cm. The last step shows a 6 cm base with sides of 2 cm and 5 cm.
See the whole ideathe big idea and the key word, in one place
The big idea
The two shorter sides of a triangle must add to more than the longest side.
Quick tests3, 4 and 5 cm: 3 + 4 = 7, more than 5, so it closes. 3, 4 and 8 cm: 3 + 4 = 7, less than 8, so it cannot.
Easy to get wrongEqual is not enough. Sides of 3, 4 and 7 cm lie flat along the base: 3 + 4 = 7 makes a straight line, not a triangle.
Key word
- longest side
- the side of a triangle with the greatest length; the other two together must be longer
Ready to use it?
- Decide whether three lengths can make a triangle.