The Definition of a Triangle

Examples of triangles Properties of triangles Classification of triangles Table of Contents Examples of TrianglesA triangle is a special kind of shape. What do you think makes a shape a triangle?

Be sure to think about this question before going on with the lesson

Properties of trianglesBefore we decide exactly what makes a shape a triangle, we need to understand what a closed figure is.

A figure is

closedif all its sides are connected.

These figures are closed. These figures are not closed. A

triangleis a closed figure with three sides.Use this definiton to figure out which of the following shapes are triangles and which are not.

Click here to check your answers.

Can you think of any triangles that you see in your everyday life?

(Hint: look at bridges or radio towers.)

Classification of TrianglesThere are many different kinds of triangles.

The following table outlines some basic types of triangles.

Acute trianlgesare triangles in which the measures of all three angles are less than 90 degrees.Obtuse trianglesare triangles in which the measure of one angle is greater than 90 degrees.Right trianglesare triangles in which the measure of one angle equals 90 degrees.Equilateral trianglesare triangles in which all three sides are the same length.Isosceles trianglesare triangles in which two of the sides are the same length.Scalene trianglesare triangles in which none of the sides are the same length.Triangles: Everything you ever wanted to know about them