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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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  • blobby green style avatar for user annie stokes
    how to perimeter of a semiircle on top of a triangle?
    (57 votes)
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    • leaf green style avatar for user Project
      annie, to find the perimeter of a semicircle on top of a triangle, you have to know 3 things:
      1) The word "semicircle" literally means "half of a circle".
      2) The formula for calculating the perimeter of a circle (Which is pi*d, where d is going to be the diameter of the circle --- or in our case the semicircle, and pi means the number π -- or 3.141592653.....)
      3) You need to realize that because the semicircle is on top of the triangle, the length of that side of the triangle is going to be the same as the diameter of the semicircle.
      Now, because of fact #1 above, all you need to do is calculate the perimeter of a circle (using facts #2 and #3) and then divide the answer by 2. In other words Perimeter of semicircle = (pi*d) ÷ 2
      If you now want to calculate the perimeter of the semicircle and the triangle together, then all you have left to do is to add the length of the other two sides of the triangle to the perimeter of the semicircle.

      I hope this information helps you to understand so that you will be able to do similar problems in the future. :)
      (22 votes)
  • piceratops seed style avatar for user Shahin Ramezani
    but what if the perimeter and area are the same
    (15 votes)
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  • aqualine ultimate style avatar for user louis.diaz
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    Problem?
    (12 votes)
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  • starky sapling style avatar for user Void_Ent1ty
    Where does the word Perimeter come from?
    (13 votes)
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  • hopper jumping style avatar for user szhao
    So for an octagon or hexagon we would just multiply by the number of the sides and the length of each side?
    (4 votes)
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    • duskpin sapling style avatar for user Rohini
      Its not always like that. You could only do that when all your sides lengths are equal. Say you have a octagon with equal length of sides = 5 , then you could find the perimeter by multiplying the number of sides by the length. In this case, 8 x 5 = 40. But if all your sides are not equal then you have to add them one by one.
      (7 votes)
  • hopper happy style avatar for user billybrookes5
    at how does the square equal 28?
    (4 votes)
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    • duskpin ultimate style avatar for user Rossa
      Well, the question was asking for the perimeter of the square.
      A square's sides are all congruent.
      A formula for perimeter for * squares* is 4 s_ (with _s being the side length).
      The problem states that one side is equal to 7 m. So in the equation, replace s with 7.
      So the equation is now 4(7) = P (P is perimeter)
      4 times 7 = 28 m, and that's how they got 28 m.
      (6 votes)
  • blobby green style avatar for user 240480
    why is it called a perimeter?
    (4 votes)
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  • starky sapling style avatar for user LANDON267
    What if its a paralellogram? how would i find the area?
    (5 votes)
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  • duskpin sapling style avatar for user Angelina Snow
    It's sort of long to count squares - Is there a straightforward way?
    (3 votes)
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    • aqualine ultimate style avatar for user Lanz Sidney Povey
      Whenever you see a compound shape (one with multiple shapes attached to it), then you should multiply the length by the width.

      Looking at the example in the video [ - ], the compound shape can be broken up into 3 shapes:
      1) 2 by 5 rectangle (4 + 2 + 5 + 1 = 12)
      2) 1 by 2 rectangle (2 + 2 + 1 = 5)
      3) 1 by 3 rectangle (3 + 3 + 1= 7)

      Then add all of those up and you should get the 24 squares, the same answer that you would have gotten if you were to count all of them the long way.
      (5 votes)
  • aqualine ultimate style avatar for user Sofia
    Where does the word Perimeter come from?
    (4 votes)
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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.