Perimeter of a Triangle

Lesson Objective

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

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

a triangle with side lengths of a,b and 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

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Video on the examples on using the formula P=a+b+c

Math Videos Transcript

PART 1


00:00:04.030
In this lesson, we will learn about the perimeter of a triangle.

00:00:09.110
Consider this triangle. This triangle has the side lengths of ‘a’, ‘b’ and ‘c’.

00:00:19.000
Now, to find the perimeter of this triangle, P, we just add the length of these sides together.

00:00:26.160
Hence, we have ‘a’ plus 'b' and finally, plus 'c'.

00:00:38.100
Therefore, the perimeter, 'P', of a triangle, is equals to a + b + c.

00:00:47.020
Note that, in any calculation, when we found the value of 'P', it is important to include the unit.

00:00:55.000
For example, if 'a', ‘b', and 'b' are given in centimeter, the unit for the perimeter will be in centimeter as well.

00:01:05.050
The next video will show some examples on using this formula.


PART 2

00:00:04.020
After understanding the basics behind the perimeter of a triangle, let's see some examples on it.

00:00:12.010
Find the perimeter of this triangle, when its side lengths are 3cm, 4cm, and 5cm.

00:00:22.090
We start with the formula for the perimeter of a triangle, 'P', equals to a + b + c.

00:00:30.180
One of the sides is given as 3cm. Hence, we can substitute 'a' with 3.

00:00:38.200
Similarly, we can substitute 'b' with '4', and substitute 'c' with, 5.

00:00:47.130
Let's simplify this. 4 add with 5, gives 9. 3 add with 9, gives 12.

00:00:59.130
Now, we have 'P' equals to 12.

00:01:04.010
Note that, this number has no meaning, unless we include the unit for it.

00:01:10.030
Since these lengths are given in centimeter, the unit for the perimeter will also be in centimeter.

00:01:17.070
Therefore, the perimeter of this triangle is equals to 12 centimeter.

00:01:24.130
Next example, the perimeter of this triangle, is 21ft. Its side lengths are 7ft, 9ft, and x feet. Find x.

00:01:40.070
Again, we start with the formula for the perimeter of a triangle, 'P' equals to, a + b + c.

00:01:49.190
Since the perimeter is given as 21ft, we can substitute 'P' with 21.  

00:01:57.090
Now, the length of one of the side is given as x. Hence we can substitute ‘a’ with x.

00:02:06.090
Similarly, we can substitute 'b' with 7, and 'c' with 9.

00:02:15.090
We can simplify this equation, by adding 7, with 9. This gives 16.

00:02:22.210
Now, we have, x + 16, equals to, 21.

00:02:29.080
Let's rewrite this, so that it will look neater.

00:02:33.200
Here, note that, to find x, we need to remove 16.

00:02:39.230
We can do so, by adding -16, to both sides of the equation. Hence, we get, x equals to, 21 minus 16.

00:02:53.060
21, minus 16, give 5.

00:02:57.210
Now, we have x, equals to 5. Note that, it is important to include the unit.

00:03:07.130
From here, it is obvious that, we have 5 feet.  

00:03:12.200
Finally, x = 5 feet.

00:03:18.010
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

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