why the formula of the area of the circle is taken a pi*r^2?

I was wondering about this as well but luckily for us Sal made a video that gives an intuition about this formula: https://www.khanacademy.org/math/geometry/hs-geo-foundations/hs-geo-area/v/informal-argument-for-the-area-of-circle-formula

so you multiply pi by the radius squared to get circumference, please tell me if im wrong.

no what you are telling is the formula for area of a circle. the formula for circumference of a circle is 2 x pi x r(radius). Hope your confusion is clear now.

Is it possible to calculate the perimeter of a sphere? We had done perimeter of a rectangular prism in class, but wanted to find out for sphere. Thanks to anyone who answers.

Well you could find the diameter, radius, and surface area of a sphere, but that usually comes later. Here is the formula if you're curious... A = 4 π r^2 Hope this helps!

how did he go from radius squared=36 to radius=6 in the video ? He said something about the principles or whatever so does that apply for any/every problem like this? please answer back!

So opposite of squaring is square root, x= ±√36, when we take the square root we can get a positive or negative answer x=6 or x=-6. However, in many practical problems (such as finding the radius), we know we cannot have a negative distance for radius, so we only pick the positive (6)

How come I get REALLY CONFUSED when watching these videos? And, I can't understand anything y'all be talking bout.

yeah, i get confused and have to watch them twice. but i just try and find the formula and write it down that way i can aply it to my work

At 0:09, what do you mean by "pie"?I 'm not getting this.

He's talking about the number pi, which is approximately 3.141593. This video introduces pi: https://www.khanacademy.org/math/geometry/cc-geometry-circles/circle-basics/v/circles-radius-diameter-and-circumference

Can you solve this question with π=23/7

Yes. All you have to do is divide 23 by 7 and you have your answer (pi=3.28571428571, which is not correct because the simplification of pi is 3.14)

Main content

7th grade

Course: 7th grade > Unit 9

Lesson 2: Area and circumference challenge problems

Finding circumference of a circle when given the area

Name: Finding circumference of a circle when given the area
Uploaded: 2013-10-26T05:47:24Z
Description: Learn how to find the circumference, the distance around a circle, when given the area.

Google Classroom

Learn how to find the circumference, the distance around a circle, when given the area. Created by Sal Khan.

Want to join the conversation?

Sort by:

SAKSHAM
Posted 10 years ago. Direct link to SAKSHAM's post “why the formula of the ar...”
why the formula of the area of the circle is taken a pi*r^2?
Button navigates to signup pageComment on SAKSHAM's post “why the formula of the ar...”
(7 votes)
Answer
- Brian da Silva
  Posted 6 years ago. Direct link to Brian da Silva's post “I was wondering about thi...”
  I was wondering about this as well but luckily for us Sal made a video that gives an intuition about this formula:
  https://www.khanacademy.org/math/geometry/hs-geo-foundations/hs-geo-area/v/informal-argument-for-the-area-of-circle-formula
  Comment on Brian da Silva's post “I was wondering about thi...”
  (11 votes)
Liam
Posted 9 years ago. Direct link to Liam's post “so you multiply pi by the...”
so you multiply pi by the radius squared to get circumference, please tell me if im wrong.
Button navigates to signup pageComment on Liam's post “so you multiply pi by the...”
(7 votes)
Answer
- Abhishek
  Posted 8 years ago. Direct link to Abhishek's post “no what you are telling i...”
  no what you are telling is the formula for area of a circle. the formula for circumference of a circle is 2 x pi x r(radius).
  Hope your confusion is clear now.
  Comment on Abhishek's post “no what you are telling i...”
  (6 votes)
Ashley T
Posted 8 years ago. Direct link to Ashley T's post “Is it possible to calcula...”
Is it possible to calculate the perimeter of a sphere?
We had done perimeter of a rectangular prism in class, but wanted to find out for sphere. Thanks to anyone who answers.
Button navigates to signup pageButton navigates to signup page
(6 votes)
Answer
- mcandlish19
  Posted 8 years ago. Direct link to mcandlish19's post “Well you could find the d...”
  Well you could find the diameter, radius, and surface area of a sphere, but that usually comes later.
  
  Here is the formula if you're curious...
  
  A = 4 π r^2
  
  Hope this helps!
  Comment on mcandlish19's post “Well you could find the d...”
  (4 votes)
E.O
Posted 3 years ago. Direct link to E.O's post “how did he go from radius...”
how did he go from radius squared=36 to radius=6 in the video ? He said something about the principles or whatever so does that apply for any/every problem like this?
please answer back!
Button navigates to signup pageComment on E.O's post “how did he go from radius...”
(5 votes)
Answer
- David Severin
  Posted 3 years ago. Direct link to David Severin's post “So opposite of squaring i...”
  So opposite of squaring is square root, x= ±√36, when we take the square root we can get a positive or negative answer x=6 or x=-6. However, in many practical problems (such as finding the radius), we know we cannot have a negative distance for radius, so we only pick the positive (6)
  Comment on David Severin's post “So opposite of squaring i...”
  (5 votes)
Otto16Gaming
Posted 7 months ago. Direct link to Otto16Gaming's post “his voice sounds like my ...”
his voice sounds like my old math teacher
Button navigates to signup pageButton navigates to signup page
(7 votes)
Answer
Elisabeth BigWolf
Posted 9 years ago. Direct link to Elisabeth BigWolf's post “How come I get REALLY CON...”
How come I get REALLY CONFUSED when watching these videos? And, I can't understand anything y'all be talking bout.
Button navigates to signup pageButton navigates to signup page
(4 votes)
Answer
- ZeeBeetatertot
  Posted 3 years ago. Direct link to ZeeBeetatertot's post “yeah, i get confused and ...”
  yeah, i get confused and have to watch them twice. but i just try and find the formula and write it down that way i can aply it to my work
  Button navigates to signup page
  (3 votes)
Andrew
Posted 8 years ago. Direct link to Andrew's post “At 0:09, what do you mean...”
At
0:09
, what do you mean by "pie"?I 'm not getting this.
Button navigates to signup pageComment on Andrew's post “At 0:09, what do you mean...”
(4 votes)
Answer
- ZeroFK
  Posted 8 years ago. Direct link to ZeroFK's post “He's talking about the nu...”
  He's talking about the number pi, which is approximately 3.141593. This video introduces pi: https://www.khanacademy.org/math/geometry/cc-geometry-circles/circle-basics/v/circles-radius-diameter-and-circumference
  Comment on ZeroFK's post “He's talking about the nu...”
  (3 votes)
Anderson Le
Posted 5 years ago. Direct link to Anderson Le's post “How would you find the ar...”
How would you find the area of a circle when given the circumference? I am struggling on this one problem. It states:
"A sumo wrestling ring is circular and has a circumference of 4.6pi meters."

"What is the area A of the sumo wrestling ring in square meters?"
"Give your answer in terms of pi."
Button navigates to signup pageComment on Anderson Le's post “How would you find the ar...”
(5 votes)
Answer
ayeshajagot
Posted 8 years ago. Direct link to ayeshajagot's post “Can you solve this questi...”
Can you solve this question with π=23/7
Button navigates to signup pageButton navigates to signup page
(3 votes)
Answer
- Kylee
  Posted 8 years ago. Direct link to Kylee's post “Yes. All you have to do i...”
  Yes. All you have to do is divide 23 by 7 and you have your answer (pi=3.28571428571, which is not correct because the simplification of pi is 3.14)
  Button navigates to signup page
  (2 votes)
cadetrubino11
Posted 7 years ago. Direct link to cadetrubino11's post “I'm looking for "How to f...”
I'm looking for "How to figure out the circumference of a Circle" by knowing the diameter. I can't figure out how, such and such to figure out the circumference by knowing the diameter.
Button navigates to signup pageButton navigates to signup page
(3 votes)
Answer
- 1867673
  Posted 3 months ago. Direct link to 1867673's post “If you know the diameter,...”
  If you know the diameter, then you would multiply the diameter by pi. Pi is 3.14, so if the diameter was 3, then the circumference would be 9.42. I would appreciate an upvote, as I'm trying to get a badge. :)))
  Button navigates to signup page
  (0 votes)

Video transcript

If we know some circle has an area of 36pi-- so it has an area of 36pi-- can we figure out what the circumference of this circle is? And I encourage you to pause this video, and try to think about that question. Well, from the area, we could figure out what the radius is, and then from that radius, we can figure out what its circumference is. So we know that the area, which is 36pi, is equal to pi r squared. And so if you look at it on both sides of this equation, if we divide-- let me rewrite it so it's a little bit clearer in a different color. So we could set up an equation pi r squared is equal to 36pi. Now, if we want to solve for the radius the first thing that we might want to do is divide both sides by pi. Then, we're left with r squared is equal to 36. Now, if we just solve this as a pure math equation, you might say, OK, we could take the positive and negative square root of 36. r could be plus or minus 6, but we need to remember that r is a distance, so we only care about the positive. So if we take the principal root of 36, we get r is equal to 6. From there, we can use this to figure out the circumference. So the circumference is equal to 2 pi r. Circumference is equal to 2 pi r. And in this case, r is equal to 6. So it's equal to 2 pi times 6, which is going to be equal to 12pi. So that's straightforward, area 36pi, we leverage pi r squared to figure out that the radius was 6, and then from that we were able to figure out that the circumference was 12pi.