The LessonAn 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 DefinitionThe Merriam-Webster dictionary defines an isosceles triangle as "a triangle in which two sides have the same length."
Real Examples of Isosceles TrianglesBelow 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 TriangleAn 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 TrianglesAn 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 TriangleTwo 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
- 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!