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### Course: 3rd grade > Unit 11

Lesson 1: Perimeter- Perimeter: introduction
- Perimeter of a shape
- Find perimeter by counting unit squares
- Find perimeter by counting units
- Finding perimeter when a side length is missing
- Find perimeter when given side lengths
- Finding missing side length when given perimeter
- Perimeter review

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# Perimeter: introduction

Perimeter is a math concept that measures the total length around the outside of a shape. To find the perimeter, you add together the lengths of all the sides. This works for any shape, including triangles, rectangles, pentagons, and even irregular polygons. Created by Sal Khan.

## Want to join the conversation?

- At1:51, what is the meaning of gnus?(10 votes)
- Gnus: Imaginary shapes made by Sal Khan.(20 votes)

- This is strange, but why is sometimes the perimeter bigger than the area?(10 votes)
- Perimeter and area are measured in two different units. One isn't bigger or smaller than the other because it is like comparing apples and oranges. perimeter measures length, think of steps in a line. Area measure squares of a certain length as in tiles on a floor.(2 votes)

- Why do you have to add up all the three sides?(6 votes)
- The perimeter is defined that way! The perimeter is defined as a sum of all the side lengths of a shape.(3 votes)

- What's the difference between perimeter and area? Is there really a difference? My teacher always says: Area = Length x Width = Area. But what about perimeter?(2 votes)
- Absolutely, there is a difference!

The perimeter is like a fence around a shape. It's the total distance around the outside.

On the other hand, the area is like the grass inside the fence. It's the total space inside the shape.(5 votes)

- What figures can you find the perimeter of and what are the formulas?(0 votes)
- You can find the perimeter of any 2D shape. You can't get the perimeter of 3D shapes for obvious reasons, instead we call the area of the outside of a 3D shape "
**surface area**".

Perimeter doesn't really have a formula, you just need to add up the total sum of the side lengths of a polygon. There are ways to solve for sides of a polygon, however since there are infinite polygons, there are infinite formulas, which means that I can't put them all here. (sorry!) Some famous formulas are the*Pythagorean Theorem*, for finding the hypotenuse of a triangle given the other two sides, and the*laws of sines and cosines*, which you will learn more about in**Geometry**.

Hope this helped!(0 votes)

- Do 3 dimension objects like cubes have perimeter?(26 votes)
- Perimeter is 2D/2-dimensional shapes ONLY.(7 votes)

- Perimeter is the outside length/measurement of the shape. (For example, if a square's singulair side length is 2m, that you would add: 2+2+2+2= 8 to get the perimeter - since perimeter is the the path that surrounds the shape. I'm not sure if this makes it more confusing for you, but I hope it helps.)(1 vote)

- What's the difference between a irregular object and regular object?(2 votes)
- Usually in geometry a regular shape is a shape where every side and angle is congruent, which means the same. So a regular pentagon has 5 sides of the same length. This is helpful in finding the perimeter.(2 votes)

- What does boundary mean ?(2 votes)
- boundary is basically perimeter just that boundary is another way to say perimeter(2 votes)

- Could you find the area using the numbers on the shapes also?(2 votes)
- Yes. for example, if we have a rectangle, and the measurements are "16 "11 "16 "11, you would take a "16 and an "11 and multiply those, which would be 176, which would bring your answer to a final state of 176 inches squared. hope this was of any help to you :)(1 vote)

## Video 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.