# What Is a Triangle?

## 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°).

## 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.

### Interior Angles of a Triangle

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