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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. :)
• but what if the perimeter and area are the same
• Where does the word Perimeter come from?
• the word is pronounced perimeter because peri-means around and the word meter-is referring to measurement so they put it all together and it makes perimeter.
have a blessed day today

have a blessed day
(1 vote)
• So for an octagon or hexagon we would just multiply by the number of the sides and the length of each side?
• 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.
• why is it called a perimeter?
• “Peri-“ means “around”, and “meter” refers to measurement. That makes sense because the perimeter is the measure around a shape.

I hope this helps!
• So perimeter is basically adding up al the sides of the shape?
• Yes, perimeter is the sum of the side lengths.
• 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.
• What if its a paralellogram? how would i find the area?