**About the
Triangles Introduction
Lesson**

**Summary**

This is an introductory lesson about triangles and their classification.

**Objectives**

• to introduce the concept of a triangle;

•
to show how to identify a triangle by its internal angles;

•
to show how to identify a triangle a by the length of its sides.

**Suggested Grades**

2nd Grade

3rd Grade

4th Grade

**Excerpt **

*
A triangle is one of the basic shapes of geometry. A triangle has three sides or edges. Based on the lengths of their sides, there are three different ways that triangles can be classified.*

An isosceles triangle has two sides that are equal in length. An isosceles triangle will also have two equal angles. The equal angles will be opposite the two equal sides. An isosceles triangle would look similar to this:

If the sides of a triangle are all equal, the triangle is an equilateral triangle. The internal angles of an equilateral triangle are also equal. Each internal angle is 60 degrees. An equilateral triangle would look similar to this:

*Continued...*