### Perimeter of a Triangle #### Lesson Objective

In this lesson, we will learn about the perimeter of a triangle... In this lesson, we will:

• Learn about the formula for the perimeter of a triangle.
• See an example on using the formula to find a triangle's perimeter.
• See another example on using the formula to find the side length of a triangle.

The study tips and math video below will explain more. ### Study Tips #### Tip #1

The triangle shown on the right has the side lengths of a, b and c. Its perimeter, P, will be the sum of all the side lengths. Hence:

P = a + b + c

The math video below will give more explanation on this. Also, we will see some examples on how to use this formula. ### Math Video #### Video on the perimeter of a triangle #### Math Video Transcript

PART 1

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In this lesson, we will learn about the perimeter of a triangle.

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Consider this triangle. This triangle has the side lengths of ‘a’, ‘b’ and ‘c’.

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Now, to find the perimeter of this triangle, P, we just add the length of these sides together.

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Hence, we have ‘a’ plus 'b' and finally, plus 'c'.

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Therefore, the perimeter, 'P', of a triangle, is equals to a + b + c.

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Note that, in any calculation, when we found the value of 'P', it is important to include the unit.

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For example, if 'a', ‘b', and 'b' are given in centimeter, the unit for the perimeter will be in centimeter as well.

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The next video will show some examples on using this formula. #### Video on the examples on using the formula P=a+b+c #### Math Video Transcript

PART 2

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After understanding the basics behind the perimeter of a triangle, let's see some examples on it.

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Find the perimeter of this triangle, when its side lengths are 3cm, 4cm, and 5cm.

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We start with the formula for the perimeter of a triangle, 'P', equals to a + b + c.

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One of the sides is given as 3cm. Hence, we can substitute 'a' with 3.

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Similarly, we can substitute 'b' with '4', and substitute 'c' with, 5.

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Let's simplify this. 4 add with 5, gives 9. 3 add with 9, gives 12.

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Now, we have 'P' equals to 12.

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Note that, this number has no meaning, unless we include the unit for it.

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Since these lengths are given in centimeter, the unit for the perimeter will also be in centimeter.

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Therefore, the perimeter of this triangle is equals to 12 centimeter.

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Next example, the perimeter of this triangle, is 21ft. Its side lengths are 7ft, 9ft, and x feet. Find x.

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Again, we start with the formula for the perimeter of a triangle, 'P' equals to, a + b + c.

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Since the perimeter is given as 21ft, we can substitute 'P' with 21.

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Now, the length of one of the side is given as x. Hence we can substitute ‘a’ with x.

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Similarly, we can substitute 'b' with 7, and 'c' with 9.

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We can simplify this equation, by adding 7, with 9. This gives 16.

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Now, we have, x + 16, equals to, 21.

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Let's rewrite this, so that it will look neater.

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Here, note that, to find x, we need to remove 16.

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We can do so, by adding -16, to both sides of the equation. Hence, we get, x equals to, 21 minus 16.

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21, minus 16, give 5.

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Now, we have x, equals to 5. Note that, it is important to include the unit.

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From here, it is obvious that, we have 5 feet.

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Finally, x = 5 feet.

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That is all for this lesson. Try out the practice question to further your understanding.

### Practice Questions & More #### Multiple Choice Questions (MCQ)

Now, let's try some MCQ questions to understand this lesson better.

You can start by going through the series of questions on the perimeter of a triangle or pick your choice of question below.

• Question 1 on finding the perimeter of a triangle
• Question 2 on finding a side length of a triangle #### Site-Search and Q&A Library

Please feel free to visit the Q&A Library. You can read the Q&As listed in any of the available categories such as Algebra, Graphs, Exponents and more. Also, you can submit math question, share or give comments there. 