The Lesson

A triangle is a 2-dimensional shape with three sides and three angles.

Dictionary Definition

The Oxford English Dictionary defines a triangle as "a figure (usually, a plane rectilineal figure) having three angles and three sides."

Types of Triangle

There are different types of triangle. Triangles can be classified by lengths of sides or by angles.

By Lengths of Sides

Triangles can be classified as equilateral, isosceles or scalene depending on whether 3, 2 or 0 side lengths and angles are equal.

By Angles

Triangles can be classified as acute, obtuse or right triangles depending on whether all angles are acute (less than 90°), one angle is obtuse (more than 90°) or one angle is a right angle (90°).


Read more about types of triangle

Interactive Game on the Types of Triangle

Here is an interactive game to help you learn about the types of triangle.

Properties of Triangles

Area of a Triangle

The area of a triangle is found using the formula:

In this formula, b is the length of the base of the triangle and h is the height of the triangle.
Read more about how to find the area of a triangle

Interior Angles of a Triangle

The interior angles of a triangle add up to 180°.


Read more about the interior angles of a triangle

Interactive Widget

Here is an interactive widget to help you learn about triangles.

What's in a Name?

A triangle has three corners and so three angles. Just as a tricycle has three wheels, and a tripod has three legs, a shape with three angles in called a triangle. An alternative name for a triangle is a 'trigon', as it is a three-sided polygon. This is where we get the word trigonometry.

Oblique Triangles

A triangle without a right angle is called an oblique triangle. Acute triangles and obtuse triangles are both oblique triangles.