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Unit 3: Lesson 5

Lesson 8: Relating area to circumference

Area of a circle

The area of a circle is pi times the radius squared (A = π r²). Learn how to use this formula to find the area of a circle when given the diameter. Created by Sal Khan and Monterey Institute for Technology and Education.

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• Where did the name pi originate from?
• Although the constant that π represents has been known for thousands of years, it has been depicted by various symbols.

There is some uncertainty about who was first person to use π for this constant. The earliest confirmed usage was by William Jones in 1706. However, it is quite possible that John Machin might have been the first to use it a few years earlier than William Jones.
• question:how many millimeters are in a meter? also, how many meters are in a kilometer?
• There are 1,000 millimeters in a meter, and 1,000 meters in a kilometer. This tells us that there are 1,000,000 millimeters in a kilometer!

Have a blessed, wonderful day!
• Find the area of a circle with a circumference of 12.56
How should I do this problem i have been stuck on it forever and the videos aren't helping
• The circumference of a circle and the area of a circle have one thing in common: the radius.
If I were you I'd do it in two steps:
- first calculate the radius using the circumference formula,
- then calculate the area using the radius you just calculated :-)
Hope that helps!
• How do i type the pi symbol on the keybord?
• I usually use Alt key codes, such as, Alt + 227 = π. Try it, it's easy.
(1 vote)
• No, you do not that one time you square the radius will the only time you will square it.
• What's the signficance of squaring mm? Is there a difference between mm and mm2?

Thanks.
• The same difference as betweem m and m2: mm (and m) is a unit of length, and mm2 (and m2) is a unit of area
• if you know the area how do to find the circumference, diameter and radius
(1 vote)
• Area = Pi*R^2

To find the radius (R), divide the area by Pi, then take the square root.

Once you have the radius (R), you can use it to find diameter and circumference.
Diameter = 2*R
Circumference = 2*Pi*R

Hope this helps.
• how many number were discovered foe pie?
• How could you remember all the digits of pi with out looking on a piece of paper
• That is impossible. π has infinitely many digits. It is both irrational and transcendental. π never repeats and it never ends. To date, π has been computed to 10000000000050 digits.

At this level of study (and beyond) it is customary just to report your answer in terms of π. Thus, if you had an area that was 4.2π cm², then that is how you would normally report it. You would not typically report it as something like 13.19 cm².

If you do need an easily remembered estimate of π, the most useful one is 355/113 which is accurate to seven decimal places. That is 99.9999915% accurate, whereas 22/7 is only 99.95975% accurate.)