About this video
This video explains that perimeter is the length of the boundary around a shape. It finds the perimeter of a triangle, a rectangle, a pentagon and an irregular polygon by adding up the lengths of their sides, and shows that equal sides can be multiplied instead.
You'll be able to
- explain that perimeter is the distance all the way around a shape.
- find a perimeter by adding the lengths of all the sides.
- use multiplication when all the sides are the same length.
Key words
- perimeter
- The total length of the boundary all the way around a shape.
- boundary
- The outside edge of a shape.
- pentagon
- A flat shape with five sides.
- polygon
- A flat shape made of straight sides joined together.
- unit
- The measure we count in, like feet or meters.
Transcript
When people use the word "perimeter" in everyday language, they're talking about the boundary of some area. And when we talk about perimeter in math, we're talking about a related idea. But now we're not just talking about the boundary. We're actually talking about the length of the boundary.
How far do you have to go around the boundary to essentially go completely around the figure, completely go around the area? So let's look at this first triangle right over here. It has three sides. That's why it's a triangle. So what's its perimeter? Well, here, all the sides are the same, so the perimeter for this triangle is going to be 4 plus 4 plus 4, and whatever units this is.
If this is 4 feet, 4 feet and 4 feet, then it would be 4 feet plus 4 feet plus 4 feet is equal to 12 feet. Now, I encourage you to now pause the video and figure out the parameters of these three figures. Well, it's the exact same idea. We would just add the lengths of the sides.
So let's say that these distances, let's say they're in meters. So let's say this is 3 meters, and this is also 3 meters. This is a rectangle here, so this is 5 meters. This is also 5 meters. So what is the perimeter of this rectangle going to be? What is the distance around the rectangle that bounds this area?
Well, it's going to be 3 plus 5 plus 3 plus 5, which is equal to-- let's see, that's 3 plus 3 is 6, plus 5 plus 5 is 10. So that is equal to 16. And if we're saying these are all in meters, these are all in meters, then it's going to be 16 meters. Now, what about this pentagon?
Let's say that each of these sides are 2-- and I'll make up a unit here. Let's say they're 2 gnus. That's a new unit of distance that I've just invented-- 2 gnus. So what is the perimeter of this pentagon in gnus? Well, it's 2 plus 2 plus 2 plus 2 plus 2 gnus.
Or we're essentially taking 1, 2, 3, 4, 5 sides. Each have a length of 2 gnus. So the perimeter here, we could add 2 repeatedly five times. Or you could just say this is 5 times 2 gnus, which is equal to 10 gnus, where gnu is a completely made-up unit of length that I just made up.
Here we have a more irregular polygon, but same exact idea. How would you figure out its perimeter? Well, you just add up the lengths of its sides. And here I'll just do it unitless. I'll just assume that this is some generic units. And here the perimeter will be 1 plus 4 plus 2 plus 2-- let me scroll over to the right a little bit-- plus 4 plus 6.
So what is this going to be equal to? 1 plus 4 is 5, plus 2 is 7, plus 2 is 9, plus 4 is 13, plus 6 is 19. So this figure has a perimeter of 19 in whatever units these distances are actually given.
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