Triangles and Their Sides

Lesson Plan - Get It!

Audio:

A triangle is any polygon with three angles and three straight lines.

Were you able to tell the difference between the triangles pictured above? You should have observed that the lines used in each triangle are very different.

The left triangle is composed of lines that are all the same length, whereas the right triangle is composed of lines that are all different lengths. You also may have noticed that each triangle has different angles. The left triangle has angles that are all the same, and the right triangle has angles that are all different.

There are two ways to classify triangles; by their sides and by their angles. In this lesson, you will learn how to classify and name triangles by their sides.

There are three terms for classifying triangles by their sides: equilateral triangle, isosceles triangle, and scalene triangle.

Based on the name, what do you think the sides of an equilateral triangle look like? Tell your teacher or parent. The sides of an equilateral triangle are all equal in length. Get out a ruler and measure each side of the equilateral triangle pictured below. Are all the sides the same length?

The next type of triangle is an isosceles triangle. Look at the example below and describe the sides of the triangle to your teacher or parent. An isosceles triangle has two sides that are the same length and one side that is a different length. Show your teacher or parent which sides of the triangle are the same and which side is different.

There is a fun trick you can use to help you quickly identify an isosceles triangle. In the middle of the word "isosceles" are the letters "sos." Think of these letters as representing the phrase, "same, other, same." Then, apply this phrase to an isosceles triangle by writing an "s" next to each side that is the same length, and an "o" next to the side that is different. By doing this, you can easily see that two sides are the same and one side has a length that is not the same.

The final type of triangle is a scalene triangle. A scalene triangle has no sides that are equal in length. In other words, all three sides are different. Use your ruler to measure each side of the triangle below. Are any of the sides the same?

There are three ways to describe triangles based on their sides. Tell your teacher or parent the three classifications and define each one. Then, move on to the Got It? section to practice identifying each type of triangle.