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Finding the Perimeter of Polygons

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  • 0:06 Polygons
  • 0:41 Perimeter
  • 0:56 Finding the Perimeter
  • 3:11 Regular Polygons
  • 4:21 Lesson Summary
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

In this video lesson, you will learn the process you need to take to find the perimeter of polygons. You will also learn that there is a shortcut if your polygon is a regular polygon.

Polygons

Polygons, flat shapes with straight sides, are very popular. Look around, and you are bound to see some. Your computer monitor is most likely a polygon, a rectangle to be exact. And just look at your window, too. That's also a polygon; probably a rectangle as well. And if you look down on a neighborhood from somewhere high up, you will see that each individual property has a fence or boundary around it, and that is most likely a polygon, too. Most properties have straight lines as boundaries all around.

Perimeter

If you can picture your polygon as a type of boundary, then your perimeter is the distance all around the shape. Another way to think of perimeter is the distance it will take to walk all around the shape or boundary.

Finding the Perimeter

When you are trying to find the perimeter, the length of each side is given to you. Usually, you are given a shape and then you see that each side has been labeled with a number. Because the perimeter is the distance all around, you would start with one side and work your way around the shape until you have added up all the numbers together. This sum is your answer for the perimeter.

Let's try finding the perimeter of this fenced-in field below.

Fenced-in field for example problem
image of fence with horses

If you look down from above, you can see that the fence makes a polygon shape since all of its sides are straight. You see horses enjoying themselves in the sun. You want to make a similar kind of field on your ranch, so you need to know how much fence is needed by finding the perimeter.

Because polygons come in all kinds of shapes with any number of sides, we need to make sure that we know the lengths of all of our sides. So, first, I look at my diagram, and I look for all my sides. I do see that they are all labeled with their lengths:

To find perimeter, you need to know the lengths of all your sides.
fence with labeled sides

I have all the information I need to proceed. I pick the top side as my starting point, and I'm going to work my way clockwise around the shape. I need to add all of my numbers, so I start writing 8 + 5 + 5 + 8 + 11. I have all my sides written down.

Now, I double check by counting the amount of numbers I've written down and the amount of sides I have. I have five sides, and I have five numbers written down. My numbers are also correctly written down. So, now I can continue to find my answer by adding it all up. So, 8 + 5 + 5 + 8 + 11 = 37. But wait, what are my measuring units? Well, since the problem didn't specify units, I'm going to leave the number as is for an answer of 37.

If the problem gave a type of measuring unit, I would make sure that my answer had the same. So, if I was given feet, my answer would have feet in it as well. If I was given meters, my answer would have meters in it.

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