# Isosceles Triangles (Mathematics Glossary)

# What Is an Isosceles Triangle?

An isosceles triangle is a triangle with two equal sides and two equal angles across from them.**Note:**The lines through the two sides of the triangle in the image above indicate they are of equal length. The two arcs indicate the two angles are equal.

#### Dictionary Definition

The Merriam-Webster dictionary defines an isosceles triangle as "a triangle in which two sides have the same length."# The Parts of an Isosceles Triangle

An isosceles triangle has 2 equal sides. These sides are called**legs**.

The third side is called the

**base**.

An isosceles triangle has 2 equal angles. These are the angles between each leg and the base. They are called

**base angles**.

The angle between the legs is called the

**vertex angle**.

# The Symmetry of Isosceles Triangles

An isosceles triangle has 1 line of symmetry. It is the line that goes through the vertex angle and the midpoint of the base.# Real Examples of Isosceles Triangles

Below are real examples of isosceles triangles.In each case

- 2 sides are the same length
- 2 angles are the same

##### Curriculum

##### Interactive Test

**show**

Here's a second test on isosceles triangles.

Here's a third test on isosceles triangles.

##### Note

# What's in a Name?

'Isosceles' comes from the Greek words 'isos', meaning 'equal', and 'skelos', meaning 'leg'.'Isosceles' means 'equal legs', as the two equal sides are known as legs.

# Interior Angles of an Isosceles Triangle

Two of the interior angles of an isosceles triangle are equal.There are two different angles in an isosceles triangle: the base angle and the vertex angle.

We also know that the interior angles of any triangle always add up to 180°.

Knowing either the base angle or the vertex angle allows the other to be found.

In words,

**Vertex angle = 180° - (2 × Base angle)**

Base angle = (180° - Vertex angle) ÷ 2

Base angle = (180° - Vertex angle) ÷ 2

# Other Types of Triangle

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

- Equilateral triangles (with 3 equal sides and angles)
- Scalene triangles (with no equal sides and angles)