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Equilateral Triangles (Mathematics Glossary)

What Is an Equilateral Triangle?

An equilateral triangle is a triangle that has three equal angles and three sides the same length.

Note: The lines through the sides of the triangle in the image above indicate they are of equal length. The arcs indicate the angles are equal.

Dictionary Definition

The Merriam-Webster dictionary defines an equilateral triangle as "a triangle in which all three sides are the same length."

The Interior Angles of an Equilateral Triangle

Each interior angle of an equilateral triangle is 60°.

The Symmetry of Equilateral Triangles

An equilateral triangle has 3 lines of symmetry.

Real Examples of Equilateral Triangles

Below are real examples of equilateral triangles.

In each case
  • All 3 sides are the same length.
  • All 3 angles are the same (60°).
Geometry Lessons
Interactive Test

Here's a second test on equilateral triangles.
Here's a third test on equilateral triangles.

What's in a Name?

'Equilateral' comes from the Latin word 'aequilateralis'.

'aequi' comes from the word 'aequalis', meaning 'equal', and 'lateralis' means 'relating to the side'.

'Equilateral' means 'equal sides'.

The Interior Angles of an Equilateral Triangle

The interior angles of any triangle add up to 180°.

Since all three angles are the same in an equilateral triangle, the 180° is divided equally between each interior angle.

This is why each angle is 60°.

Other Types of Triangle

Equilateral triangles are one type of triangle. There are also: