Triangles

A triangle is a polygon with three sides and three internal angles. One triangle can look very different from another. Look at the triangles below. Do they all look the same?

Types of triangles

Triangles can be classified according to the number of their sides that are equal in length.

Equilateral triangles: These are triangles with three equal sides. The internal angles of an equilateral triangle are equal as well. Each internal angle of an equilateral triangle measures 60 degrees.

An equilateral triangle has three equal sides.

Isosceles triangles: A triangle with two equal sides is called an isosceles triangle. Isosceles triangles also have two internal angles which are equal. In the image below, the two equal lines have a similar mark. The two equal angles are also shown in the image.

An isosceles triangle has two sides which are the same length.

Scalene triangles: These triangles have no sides that are equal in length. Each side is a different length. All internal angles are different as well. A scalene triangle is shown below.

Each side of a scalene triangle is a different length.

Triangles can also be classified according to their internal angles.

Acute triangles: These are triangles which have three acute angles (less than 90 degrees). Equilateral triangles are also acute triangles because their internal angles all measure 60 degrees. Look at the picture of an acute triangle below.

Right triangles: These triangles have one right angle (90 degrees). They are also called right angled triangles. A right angled triangle can be seen in the picture below.

Obtuse triangles: These triangles have one obtuse angle. Obtuse angles are greater than 90° but less than 180°. Look at the obtuse triangle below.