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Perimeter of a shape

To find the perimeter of a shape, you add up the lengths of all the sides. For a square or pentagon, it's even easier - you can multiply the length of one side by the number of sides to get the total perimeter. Created by Sal Khan.

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Video transcript

What is the perimeter of the shape? Each square in the grid is a 1 by 1 centimeter square. So all we have to do is add up the lengths of these blue segments right over here. And they put it on this nice grid. And each box here is 1 by 1. So let's say we start up here. We want to make sure that we only go to where we started and we don't double count. So this perimeter is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 centimeters long. So it is 24 centimeters. The other way we could have thought about this is you could have looked at the length of each of these sides. So this is 1 plus 2 plus 3 plus 1 plus 3 plus 1 plus-- what is this-- 5 plus 2 plus 4 plus 2. And you would have also gotten 24 centimeters. Let's do a couple more of these. What is the perimeter of the square? So once again, it's the length of all of the segments that define the outside boundary of the square. And by definition, a square-- all of its sides are equal. So you have 4 sides that are all 7 meters long. So you could say it's 7 meters plus 7 meters plus 7 meters plus 7 meters, or it's 4 times 7 meters or 28 meters. Let's do one more. What is the perimeter of the regular pentagon? So it's a regular pentagon, which means all of its sides have the same length. And they give us a side of one. The length of one is 2. So all of the sides have length 2. So it's going to be 2 plus 2 plus 2 plus 2 plus 2, or essentially five 2's. Or another way of thinking about it is it's 5 times 2, which is going to be 10. You have five sides of this pentagon. Each of those sides are 2 units long. So 2 units long times 5 sides is going to be 10 units.