# Lesson Video: Types of Triangles: Sides Mathematics • 3rd Grade

In this video, we will learn how to classify triangles as equilateral, isosceles, or scalene by considering the lengths of their sides.

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### Video Transcript

Types of Triangle: Sides

In this video, we will learn how to classify triangles as equilateral, isosceles, or scalene by comparing the lengths of their sides. Each of these real-life objects contains at least one triangle. Did you spot them? In this video, we’re going to learn how to identify different types of triangles, equilateral, isosceles, and scalene.

This road sign is an equilateral triangle. Let’s look more closely at the sides of an equilateral triangle. The sides of an equilateral triangle are equal in length. The beginning of the word “equilateral” sounds like equal. We use these lines to show that the sides are equal. Another way to tell that this is an equilateral triangle would be to measure the length of each side. If all of the sides are the same length, it’s an equilateral triangle.

This type of triangle is an isosceles triangle. We can identify a triangle is isosceles because two of its sides are equal in length. It’s these two sides. An isosceles triangle has two equal sides. This is a scalene triangle. All three of its sides are different lengths. None of the sides of a scalene triangle are equal.

Let’s recap what we’ve learned about our different types of triangles. An isosceles triangle has two equal sides. An equilateral triangle has three equal sides. And all three sides of a scalene triangle are different lengths. So, a scalene doesn’t have any equal sides. Let’s try some questions now to help us practice what we’ve learned about isosceles, equilateral, and scalene triangles.

Which of these is an isosceles triangle?

In this question, we’re shown two different triangles, triangle A and triangle B. And we’re asked which of them is an isosceles triangle. Let’s think about what we know about isosceles triangles. This is an isosceles triangle. An isosceles triangle has two equal sides. So, which of our two triangles is an isosceles triangle? Is it triangle A or triangle B? It’s triangle A. These two sides are equal in length. This triangle has two longer sides, which are the same length, and one shorter side. The isosceles triangle is triangle A.

How many scalene triangles are there?

In this question, we’re shown some different triangles. And we’re asked to count how many scalene triangles there are. A scalene triangle is a triangle which doesn’t have any equal sides. In other words, all three sides have different lengths. Let’s look more closely at the triangles we’re given. Let’s start with this triangle. We can see that two of its sides are equal in length. They both measure four centimeters. And the other side measures two. This is an isosceles triangle. So, we don’t need to count this one. We’re just looking for scalene triangles.

How about this triangle? One of its sides measures four centimeters, another measures one centimeter, and the third measures three centimeters. All of the sides of this triangle are different lengths. So far, we found one scalene triangle. Let’s look at this triangle now. Its first side measures four centimeters, this side measures two centimeters, and this side measures five centimeters. All of its sides are different lengths. So, this is also a scalene triangle. Now, we found two.

Let’s look more closely at this triangle. We can see that two of its sides are equal in length. One side or the base is five centimeters in length, and the other two measure three centimeters. This is an isosceles triangle. And this triangle is also an isosceles triangle. Two of its sides are equal in length. One of the sides measures three centimeters, and two of the sides measure four centimeters.

We looked closely at the lengths of the sides of each of these triangles. We know that scalene triangles have sides which are all different lengths. We counted the number of scalene triangles. There are two.

James drew an equilateral triangle with two sides whose sum equals 16 inches. Determine the length of the third side.

In this question, we’re given a triangle 𝐴𝐵𝐶. And we’re told it’s an equilateral triangle. What do we know about equilateral triangles? We know that equilaterals have three sides equal in length. We’re not given the length of any of the sides, but we are told that the sum of two sides equals 16 inches. The sum is the total we get when we add two of the sides together. If the sum of two sides equals 16 inches and all of the sides are equal in length, to find the length of one side, we need to halve 16 inches. Half of 16 is eight. So, each of the three sides measures eight inches in length. We know that all the sides in an equilateral triangle are equal in length. The length of the third side is eight inches.

Is this an equilateral, an isosceles, or a scalene triangle?

In this question, we’re shown a triangle, and we have to identify which type of triangle it is. Is this an equilateral triangle? An equilateral has three equal sides. All of its sides are equal in length. The given triangle has two sides equal in length. So, we know it’s not an equilateral triangle. Is it an isosceles triangle? We know that isosceles triangles have two sides which are equal in length. The given triangle is an isosceles triangle. We know it’s not a scalene triangle because scalene triangles have three sides all different in length. This triangle is an isosceles triangle. We know this because two of its sides are equal in length.

What have we learned in this video? We have learned how to classify triangles by the length of their sides. We’ve learned that equilateral triangles have three equal sides. We’ve learned that isosceles triangles have two equal sides. And we’ve also learned that all the sides of a scalene triangle are different lengths.