### 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.