## 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°). ## Interactive Game on the Types of Triangle  ## 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°. ## Interactive Widget 