Isosceles Triangles
(KS2, Year 6)

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

isosceles triangle 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.

equilateral triangles examples 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. isosceles_triangle_legs The third side is called the base. isosceles_triangle_base An isosceles triangle has 2 equal angles. These are the angles between each leg and the base. They are called base angles. isosceles_triangle_base_angles The angle between the legs is called the vertex angle. isosceles_triangle_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.

isosceles triangle symmetry

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.

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

isosceles triangle angles add up to 180 degrees 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!
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This page was written by Stephen Clarke.

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