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### Course: 8th grade foundations (Eureka Math/EngageNY)>Unit 5

Lesson 2: Topic B: Foundations

# 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?

• Are there any tips or tricks for remembering the equations for finding the diameter and the circumference?
• The saying for circumference and area that sticks with me is "Cherry pies delicious, apple pies are too." That is, C = πd and A = πr²
• 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
• 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
• this stuff is confusing
• Even though I am in 6th Grade, I am happy to explain this to you! Lets get started:

Suppose the circle is this:

🟠

*To find the diameter, measure the circle from its left-right points [divide the circle in half and measure the line you made, make sure the line is straight]. Suppose you get 10in, that is your diameter. Another way as explained in the video is to divide your circumference by pi, so how to get the circumference you ask? Well, follow number 2.

2. To find the circumference, multiply your diameter by pi [Such as 2πr], with "r" being the radius.

`Notes:

Sincerely, ShashrvatRB
• my brain cells: 🚶‍♀️
• girl bye 🤣
• Why does every radius in a circle has the same length? I don't get that and its really confusing.
• same what if i have 27/9
• pi is a infinite number right?
• Yes it is proven it goes on forever
• do you ever have to use the entire pi or just 3.14? Also what is pi?