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# Radius, diameter, circumference & π

Learn how the number Pi allows us to relate the radius, diameter, and circumference of a circle. Created by Sal Khan.

## Want to join the conversation?

• how do you caculate the radius, diameter, and/or circumfrence if all you have is the area?
• Divide the area by pi and then make a square root of the quotient. You will get the length of the radius. You can then use the now-uncovered length of the radius to find the diameter(r*2) and the circumference(πd).
• Can sombody give me some examples on pi and explain whait it means and how it is connected with everything?
• Pi is 3.14159 and it is the ratio of the circumference to the diameter. It can also be represented as 22/7
• how do you find area if all you have is circumference?
• divide the circumference by 3.14 or pie then divide the answer by 2 then times that answer by itself then times it by pie or 3.14 and you have the area
• Can someone give me a problem (NOT FORMULA!) I can keep calculating to keep accurately getting digits of pi? This is just out of curiosity. I know that circumference divided by diameter should equal pi, but I can never be exact when measuring it.
Also, can this problem preferably involve whole numbers?
(Sorry if I come off as picky and demanding)
• Pi actually goes on forever, so most people just use 3.14. Here are the first 100 digits of pi, according to math.com: 3.1415926535 8979323846 2643383279 5028841971 6939937510 5820974944 5923078164 0628620899 8628034825 3421170679. It also tells you the first 1000 digits if you really want to know.
• How can I prove my math ?
I want to be a pilot but I'm not good enough in math
• well all you have to do is study math alot and keep working harder. and do your best
• Are there any tips or tricks for remembering the equations for finding the diameter and the circumference?
• I think that that is a smart way to remember circumference.
• do you ever have to use the entire pi or just 3.14? Also what is pi?
• It depends on what the question is asking... 3.14 is what a lot of questions use, especially if you want to get a rounded answer. Regular pi goes on forever, and is more useful in exact answering.
Also, pi is a number that goes on forever and ever without stopping. It is irrational, and it describes the attributes of circles.
Hope this helps! If you need more explanation, please tell me and I will come! :D
#YouKhanLearnAnything
• pi is a infinite number right?
• Almost but not its just numbers not repeating and that keep going
(1 vote)
• How would you find out the area of a circle if you have only got the diameter?