What Is an Isosceles Triangle?

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."

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.

See Also

What is a triangle? What is an angle? Interior angles of a triangle