An isosceles triangle is a triangle with two equal sides and two equal angles across from them.

In each case

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,
Please tell us using

**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."## 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.

## 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**.## Properties of Isosceles Triangles

## 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.## 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**## Interactive Game on the Types of Triangle

Here is an interactive game to help you learn about**the types of triangle**.- Click on a triangle so it drops into the correct "bucket."
- In total, there are 10 triangles to clear.
- If a triangle drops into the wrong "bucket," you will lose one of your three lives.
- Good luck!

## You might also like...

geometryunderstanding equilateral trianglesunderstanding scalene trianglesunderstanding the interior angles of a triangle

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