Idea 13of 24Builds onEqual sides face equal anglesAcute, right, obtuse or straight
Name a triangle by its angles
How do a triangle's angles give it a name?
Today you'll discover
- which angle decides the name
- three names from the angles
- how to name a triangle from its degrees
Think firstAngles of 22°, 38° and 120°
Two angles here are acute and one is obtuse. Which angle decides the triangle's name?
Decide first, then press Next. Find the biggest angle first.
A long, flat triangle with angles 22°, 38° and 120°; the 120° corner is ringed and a box names it obtuse. Then a triangle with 37°, 90° and 53°, named right, and one with 41°, 56° and 83°, named acute. The last step shows a wide triangle with angles 29°, 29° and 122°.
See the whole ideathe big idea and the key word, in one place
The big idea
The biggest angle names it: under 90° is acute, 90° is right, over 90° is obtuse.
Compare themAcute triangle: all three angles under 90°. Right triangle: one angle of exactly 90°. Obtuse triangle: one angle over 90°.
Easy to get wrongDo not name it from the smallest angle. Every triangle has at least two acute angles; only the biggest one tells the kinds apart.
Key words
- acute triangle
- a triangle whose three angles are all less than 90°
- right triangle
- a triangle with one angle of exactly 90°
- obtuse triangle
- a triangle with one angle of more than 90°
Ready to use it?
- Name triangles by their angles.