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## 3rd grade

### 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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# Perimeter of a shape

CCSS.Math:

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?(57 votes)
- 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 semicirclethe 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.**and**

I hope this information helps you to understand so that you will be able to do similar problems in the future. :)(22 votes)

- but what if the perimeter and area are the same(15 votes)
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Problem?(12 votes) - Where does the word Perimeter come from?(13 votes)
- So for an octagon or hexagon we would just multiply by the number of the sides and the length of each side?(4 votes)
- 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)

- at1:20how does the square equal 28?(4 votes)
- 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)

- why is it called a perimeter?(4 votes)
- “Peri-“ means “around”, and “meter” refers to measurement. That makes sense because the perimeter is the
**measure around**a shape.

I hope this helps!(5 votes)

- What if its a paralellogram? how would i find the area?(5 votes)
- How you can find the area is length times width.(2 votes)

- It's sort of long to count squares - Is there a straightforward way?(3 votes)
- 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 [0:00-0:58], 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)

- Where does the word Perimeter come from?(4 votes)

## 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.