What is an Equilateral Triangle? - Definition, Properties & Formula

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  • 0:01 Definition
  • 0:48 Perimeter of an…
  • 1:45 The Area of an…
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Lesson Transcript
Instructor: Joseph Vigil
In this lesson, you'll discover what equilateral triangles are and the properties they have that make them unique. You'll also learn two formulas that are associated with equilateral triangles. Then, you can test your knowledge with a brief quiz.


We can split the word equilateral in two parts: equi meaning equivalent and lateral meaning side. So an equilateral triangle is simply a triangle whose three sides are all equal. Who knew English could help us in math?

Here's an example of an equilateral triangle:

all sides and angles are equal in this triangle

You'll notice that along with this triangle's sides, its three angles are also all equal. Since the sum of a triangle's angles is always 180 degrees, each angle in an equilateral triangle must measure 60 degrees. This is because we must divide 180 degrees evenly between the three angles: 180 / 3 = 60.

Perimeter of an Equilateral Triangle

The perimeter of any shape is the sum of the lengths of all its sides. Since an equilateral triangle's sides are all of equal length, we only need the length of one side to calculate its perimeter.

Once we have the length of one side, we would add it to itself three times to find the triangle's perimeter. Of course, an easier way to do this would be to multiply the side's length by three. So we can say that the formula for the perimeter of any equilateral triangle is:

P = 3s

s is the length of a side.

Let's find the perimeter of this equilateral triangle:

an equilateral triangle with one side seven inches long

The diagram tells us that one of its sides measures 7 inches. That's all we need to plug in to our formula!

P = 3s
P = 3 * 7
P = 21

This triangle's perimeter is 21 inches.

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