Idea 12of 24Builds onName a triangle by its sides
Equal sides face equal angles
If two sides of a triangle match, what else matches?
Today you'll discover
- what equal sides do to the angles
- the angles of an equilateral triangle
- how to spot equal angles
Think firstTwo sides are 5 cm
Two sides match. What do you predict about its three angles?
Decide first, then press Next. Match each ticked side with the corner across from it.
A triangle with sides 6 cm, 5 cm and 5 cm, the two 5 cm sides ticked. Its angles are then shown: 53° and 53° with double arcs, and 74°. Then an equilateral triangle with three 60° angles, and a scalene one with 41°, 56° and 83°. The last step shows a tipped, wide triangle with sides 7 cm, 4 cm and 4 cm.
See the whole ideathe big idea and the key word, in one place
The big idea
In a triangle, equal sides sit across from equal angles.
Sides and angles match up
Three equal sides: three equal angles. Two equal sides: two equal angles. No equal sides: no equal angles.
- Equilateral60°, 60°, 60°
- Isosceles53°, 53°, 74°
- Scalene41°, 56°, 83°
Easy to get wrongThe two equal angles are across from the equal sides, not between them. In an isosceles triangle they are the corners at the ends of the odd side.
Key word
- equal angles
- angles with the same number of degrees, marked with matching arcs
Ready to use it?
- Find the equal angles in an isosceles triangle.