# Triangles (Mathematics Glossary)

# 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°).

Read more about 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

##### Curriculum

##### Interactive Test

**show**

Here's a second test on the triangle.

Here's a third test on the triangle.

##### Note

# 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