# What is an Acute Triangle? - Definition, Facts & Example

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• 0:05 Definitions
• 0:29 What's Acute and What's Not?
• 2:28 Lesson Summary

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Lesson Transcript
Instructor: Joseph Vigil
In this lesson, you'll find out what makes a triangle an acute triangle. You'll also find some examples of what keeps a triangle from being acute. You'll also get to test your knowledge with a brief quiz.

## Definitions

An acute angle is any angle less than 90 degrees. So, as its name suggests, an acute triangle is a triangle whose three angles are all smaller than 90 degrees. How can you tell if a triangle is acute? Measure all the angles, and if they are less than a right angle (a 90-degree or square angle), then it's an acute triangle.

## What's Acute and What's Not?

There are other types of triangles that can be acute triangles and ones that definitely aren't. You can tell by following the definition, but there are other ways, too.

1. Sometimes you can tell by doing some simple math. You probably know that the sum of any triangle's angles is always 180 degrees. In an acute triangle, the sum of any two angles is always greater than 90 degrees. This is because in an acute triangle, no one angle can be greater than 89.999 degrees (and yes, those nines could go on forever). So the other two angles must add up to at least 180 minus 89.999 (or 90.001) degrees. If you only knew two angles, let's say 40 and 20 degrees, you know the triangle isn't acute because those two angles only add up to 60 degrees.

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