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Lesson 1: Perimeter

# 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.

## Want to join the conversation?

• how to perimeter of a semiircle on top of a triangle? • 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. :)
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Problem? • Where does the word Perimeter come from? • So for an octagon or hexagon we would just multiply by the number of the sides and the length of each side? • at how does the square equal 28? • 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.
• why is it called a perimeter? • What if its a paralellogram? how would i find the area? • It's sort of long to count squares - Is there a straightforward way? • 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. 