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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.

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  • aqualine ultimate style avatar for user Maddie
    how would you get the area and the perimeter of an circle
    (13 votes)
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    • piceratops ultimate style avatar for user Denis Orlov
      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
      (18 votes)
  • aqualine tree style avatar for user Ash Siegmundt
    At and , why didn't she just add the 4s and add 8 to the other numbers?
    (22 votes)
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  • mr pink orange style avatar for user RoninXYork
    could we find the area of a circle?
    (5 votes)
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    • primosaur seed style avatar for user Ian Pulizzotto
      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!
      (11 votes)
  • blobby green style avatar for user Tshedza Badugela
    is there no other formula to find the perimeter of the missing sides ?
    (6 votes)
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  • aqualine ultimate style avatar for user Aqualine
    I really need to know if you can find the area of a CIRCLE.
    (2 votes)
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    • starky ultimate style avatar for user Navji
      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.
      (10 votes)
  • duskpin seed style avatar for user meoke
    why did you add so weirdly? to
    (5 votes)
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  • blobby green style avatar for user BriellaF
    At I Knew It was 32
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  • leafers sapling style avatar for user Juliette BEUREUX
    743th number of pi?
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  • mr pants teal style avatar for user kinley.B
    Do you like math?
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  • blobby green style avatar for user elephant
    levi do you see my name yet?
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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.