## 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

## 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