## The Lesson

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."

## 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. ## 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  