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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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# Finding perimeter when a side length is missing

The video is teaching us how to find the perimeter of a shape. To do this, we need to measure the lengths of all the sides and add them together. Sometimes we have to use clues to figure out the length of a side if it isn't labeled. The perimeter is the total distance around the outside of the shape. Created by Lindsay Spears.

## Want to join the conversation?

- At1:16and1:14, why didn't she just add the 4s and add 8 to the other numbers?(21 votes)
- Because she wanted to add them at the end with all the other numbers so it would make sense.

That's what I think(1 vote)

- how would you get the area and the perimeter of an circle(11 votes)
- perimeter of a circle is called
**Circumference**

To find**circumference**of a circle, multiply diameter by Pi value (3.14..):

Circumference = Pi * Diameter

or if you know the radius, the formula will be be:

Circumference = Pi * Radius * 2

Because diameter = 2 * radius

To find**Area**of a circle, multiply Pi by squared radius. Formula looks like this:

Area = Pi * r²(14 votes)

- could we find the area of a circle?(6 votes)
- Good question for third grade! The answer is yes!

The radius of a circle is the distance from the center to any point on the circle.

The number pi is the unending decimal 3.14159265358979323... .

For calculations, 3.14 is a good estimate of pi.

Then the area of a circle is pi times the radius times the radius.

Have a blessed, wonderful day!(6 votes)

- I really need to know if you can find the area of a CIRCLE.(4 votes)
- Yes, it is possible. (the area of a circle is pi (3.14159) times the radius of the circle squared (squared = the number to the power of two, or the number times itself))

*pi is usually denoted by the Greek letter π, which is used to represent a constant (its value never changes) value of approximately 3.14159.

*To find the radius of a circle, we can either take the diameter of the circle (distance across the circle) and divide it by two.

Or, by taking a measurement from the center of the circle to any point on the boundary (this is the Radius)

Or, by taking the circumference (the distance around the circle) and dividing it by 2π (2 * 3.14159)

For example, if we know the radius of a circle (lets say 15cm) then we can find the area by taking pi * 15 to the power of 2 (15*15) = (3.14159 * 15^2 = 707)

So, a circle with the radius of 15cm has an area of 707cm

You'll learn more about it later in geometry.(7 votes)

- Do you like math?(6 votes)
- is there no other formula to find the perimeter of the missing sides ?(5 votes)
- hey grace i"m christain from jgs 3rd grade(4 votes)
- i love this goofy website(1 vote)
- Goofy?! This website is a lifesaver.(4 votes)

- levi do you see my name yet?(4 votes)
- I swear this is the dude with just an altered voice.(2 votes)
- SHE is definitely not SAL(3 votes)

## Video transcript

- [Voiceover] What is the
perimeter of the figure below? So down here, we have this figure, and we are asked to find the perimeter of this figure. Perimeter is the distance all the way around the outside of a shape. So in this case, if I were to walk around the outside, or maybe a human's too big, if an awesome little ant was to walk all the way around the
outside of this figure, how far she walked would be the perimeter. So to find that, what we can do is figure out how long is this side, and then combine it with
the length of this side, and combine it with the
length of this side, and once we combine all
of the side lengths, we'll have the perimeter, or the distance around
the outside of the figure. So let's start, we can start up here. We can see that this side length is five centimeters. So we have five centimeters, plus, moving down the side here, we have another three centimeters, and then going across, next distance is four centimeters. After that, we go down
this side right here, which is another four centimeters, and as we keep going, across the bottom, we have another nine centimeters. And then, we head up this side, and uh oh, we don't have a label. We don't know how far this is. But to find perimeter, we need the distance
around the entire outside. If this little ant, she walked the whole way, we've gotta know the
entire distance she walked. So what we can do is, we can look over here and say, this length right here
is three centimeters. Well then, that same length over here is also three centimeters. And where that three centimeters left off, this four centimeters, this length, picked up. So from here to here is another four centimeters. So if we have four centimeters plus another three centimeters, that's a total of seven more centimeters. So now we know the entire, all of the lengths, around the outside. So if we combine them or add them, we'll know the perimeter. So we can start, five plus three is eight. Four plus four is another eight. Nine plus seven, let's
see, instead of seven, I could do nine plus one and six, 'cause that's the same as seven. Nine plus one is 10, and 10 plus six is 16. Eight plus eight is another 16, so we have a 16 and a 16. 16, let's see, six ones plus six more ones is 12 ones, and one 10 plus another 10, or 10 plus 10 is 20. So another way we could add this is 20 plus 12, which is a total of 32. And we're talking about centimeters here, so 32 centimeters. The distance all the way around the outside of this figure, or the perimeter of the figure, is 32 centimeters.