Perimeter of a Triangle

Learn about the perimeter of a triangle.

Perimeter of a Triangle Lesson

The Perimeter Formula

A triangle is a polygon with three sides and three vertices. The formula for perimeter is given as:

P = a + b + c

Where P is the perimeter, and a, b, and c are the three side lengths.

Perimeter of a triangle
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Perimeter of a Triangle Example Problems

Let's go through a couple of example problems to practice finding the perimeter of a triangle.

Example Problem 1

What is the perimeter of a triangle with side lengths of 5, 6, and 3?


  1. Let’s plug the side lengths into the perimeter formula.
  2. P = a + b + c
  3. P = 5 + 6 + 3 = 14
  4. The perimeter of the triangle is 14.

Example Problem 2

An equilateral triangle has a perimeter measured to be 81 centimeters. What are the side lengths?


  1. Since an equilateral triangle has three equal sides, a = b = c. We can substitute a + b + c with 3a.
  2. Plugging what we know into the perimeter formula, we get:
    P = a + b + c
    81 = 3a
    a = 27, b = 27, c = 27
  3. The three sides each have a length of 27 centimeters.
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