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Lesson 2: Area and circumference challenge problems

# Finding circumference of a circle when given the area

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

## Want to join the conversation?

• why the formula of the area of the circle is taken a pi*r^2?
• 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?
• 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 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?"
• it is 4.6 divided by two to get the radius of 2.3, and then you square it to get 5.29 pi, which is le answer :)
• 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.
• 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. :)))
• 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
• Can you solve this question with π=23/7