# What Is a Triangle?

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°).

# 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.

# Interior Angles of a Triangle

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

Geometry Lessons
##### Interactive Test
show

Here's a second test on the triangle.
Here's a third test on the triangle.

# 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.

# Finding the Area of a Triangle Using Trigonometry

The standard way to find the area of a triangle is:
½ × base × height
An alternative way to calculate the area of a triangle is using trigonometry.

The area of a triangle is given by the formula:

In this formula, a and b are two sides of the triangle, and C is the angle between these sides.

Read more about how to find the area of a triangle using trigonometry