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How Are Equilateral, Isosceles and Scalene Triangles Different

Updated on July 11, 2014

Learning about angles and triangles is an important part of geometry.

There are different kinds of triangles called Equilateral, Isosceles and Scalene. Triangles have three sides. The tri in triangle means three like in tricycle. A tricycle has three wheels.

Triangles also have three angles. These are the areas colored in red on the pictures below. Angles are measured in degrees (°).The degrees in a triangle will always add up to 180°. Congruent is a word that means the same size.


An equilateral triange has 3 equal sides
An equilateral triange has 3 equal sides

Equilateral Triangles

Each of the angles on an equilateral triangle is 60°. If you add the degrees in each angle together you will get 180 degrees.

60° + 60° + 60° = 180°. The equi in equilateral means equal or the same. Lateral means side. So, all sides on an equilateral triangle are the same size. And all angles are the same size. So, all sides and angles on an equilateral triangle are congruent.


Only two angles in an isosceles triangle are congruent
Only two angles in an isosceles triangle are congruent

Isosceles Triangle

On an equilateral triangle, all the sides and angles are the same size. On an isosceles triangle, only two of the angles are the same size. One is a different size. So, only two sides and two angles on an isosceles triangle are congruent.

The pictured triangle has two 45° angles and one 90° angle. If you add all of the angles together, it will also equal 180°.

45° + 45° + 90° = 180°

The angles in a scalene triange are never congruent
The angles in a scalene triange are never congruent

Scalene triangles

On an equilateral triangle, all the sides and angles are the same size. On an isosceles triangle, two of the angles are the same size. One is a different size.

Scalene triangles have no equal or congruent sides at all. But all three angles must still add up to 180°. If you look at the degrees on the scalene triangle in the picture, you will see that the numbers are all different. But all three still add up to 180°.

55° + 80° + 45° = 180°

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