## The Lesson

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."

## 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°).

## 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.

## 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°.