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Course: Class 4>Unit 6

Lesson 5: Perimeter

Perimeter: introduction

Perimeter is a math concept that measures the total length around the outside of a shape. To find the perimeter, you add together the lengths of all the sides. This works for any shape, including triangles, rectangles, pentagons, and even irregular polygons. Created by Sal Khan.

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• Do 3 dimension objects like cubes have perimeter?
• Perimeter is 2D/2-dimensional shapes ONLY.
• Why do you have to add up all the three sides?
• The perimeter is defined that way! The perimeter is defined as a sum of all the side lengths of a shape.
• What's the difference between perimeter and area? Is there really a difference? My teacher always says: Area = Length x Width = Area. But what about perimeter?
• Absolutely, there is a difference!

The perimeter is like a fence around a shape. It's the total distance around the outside.

On the other hand, the area is like the grass inside the fence. It's the total space inside the shape.
• What figures can you find the perimeter of and what are the formulas?
• You can find the perimeter of any 2D shape. You can't get the perimeter of 3D shapes for obvious reasons, instead we call the area of the outside of a 3D shape "surface area".

Perimeter doesn't really have a formula, you just need to add up the total sum of the side lengths of a polygon. There are ways to solve for sides of a polygon, however since there are infinite polygons, there are infinite formulas, which means that I can't put them all here. (sorry!) Some famous formulas are the Pythagorean Theorem, for finding the hypotenuse of a triangle given the other two sides, and the laws of sines and cosines, which you will learn more about in Geometry.

Hope this helped!
• At , what is the meaning of gnus?
• Gnus: Imaginary shapes made by Sal Khan.
• This is strange, but why is sometimes the perimeter bigger than the area?
• Perimeter and area are measured in two different units. One isn't bigger or smaller than the other because it is like comparing apples and oranges. perimeter measures length, think of steps in a line. Area measure squares of a certain length as in tiles on a floor.
• Perimeter is the outside length/measurement of the shape. (For example, if a square's singulair side length is 2m, that you would add: 2+2+2+2= 8 to get the perimeter - since perimeter is the the path that surrounds the shape. I'm not sure if this makes it more confusing for you, but I hope it helps.)
(1 vote)
• What's the difference between a irregular object and regular object?
• Usually in geometry a regular shape is a shape where every side and angle is congruent, which means the same. So a regular pentagon has 5 sides of the same length. This is helpful in finding the perimeter.
• What does boundary mean ?
• boundary is basically perimeter just that boundary is another way to say perimeter
• What's the difference between perimeter and area? Is there really a difference? My teacher always says: Area = Length x Width = Area. But what about perimeter?
• Your teacher is talking about the Area of a rectangle shape.
The Perimeter of a rectangle shape is different.
See this illustration of a general rectangle:

Length
|‾‾‾‾‾‾‾‾‾|
Width | | Width
|_________|
Length

For perimeter imagine you are travelling around the border of the shape. Add each line length together and you get the perimeter.
Try this yourself! You'll get the formula for the perimeter of any rectangle.
Perimeter = Length + Width + Length + Width
This is the same as Perimeter = (Length + Width) * 2, so adding length and width then multiplying that by 2.

Hope it helps :D