Triangles

A three-sided polygon is a triangle. There are several different types of triangles (see diagram), including:

• Equilateral – all the sides are equal lengths, and all the internal angles are 60°.
• Isosceles – has two equal sides, with the third one a different length. Two of the internal angles are equal.
• Scalene – all three sides, and all three internal angles, are different.

Triangles can also be described in terms of their internal angles (see our page on Angles for more about naming angles). The internal angles of a triangle always add up to 180°.

A triangle with only acute internal angles is called an acute (or acute-angled) triangle. One with one obtuse angle and two acute angles is called obtuse (obtuse-angled), and one with a right angle is known as right-angled.

Each of these will also be either equilateral, isosceles, or scalene.