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### Course: Class 9 (Foundation) > Unit 11

Lesson 2: Area of a circle# 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.

## Want to join the conversation?

- is there a formula for an oval or is it impossible?(43 votes)
- The term oval is not normally used in geometry because it is not very well defined in common usage.

What you might mean by oval is known in geometry as an ellipse. And, yes, there are equations for it.

There are oval-like shapes that are studied in advanced calculus courses, but that is well beyond this level of study.(76 votes)

- Where did the name pi originate from?(23 votes)
- pi is a greek letter(52 votes)

- If you are already squaring the radius then why do you have squared in your final answer?(23 votes)
- No, you do not that one time you square the radius will the only time you will square it.(3 votes)

- question:how many millimeters are in a meter? also, how many meters are in a kilometer?(17 votes)
- 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!(16 votes)

- 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(8 votes)- 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!(11 votes)

- How do i type the pi symbol on the keybord?(7 votes)
- I usually use Alt key codes, such as, Alt + 227 = π. Try it, it's easy.(10 votes)

- what is a circumfrence?(1 vote)
- If you were to put it in other math forms, the perimeter of the circle.(12 votes)

- Why do we use pi to solve the area of circles and circumferences and such (if area and circumference aren't the only one). Is their something else that could possibly help? (other than 22/7 as well). Could tau be used?(0 votes)
- Tau is an other form as well. 1 Tau is equal to 1 complete revolution around a unit circle. So yes, Tau can be used in substitution for pi. Pi is used because you may not see it yet but in Calculus it will be easier to use pi instead of tau. Research and find a definite answer, good luck!(33 votes)

- 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.(13 votes)

- What's the signficance of squaring mm? Is there a difference between mm and mm2?

Thanks.(5 votes)- 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(5 votes)

## Video transcript

A candy machine creates
small chocolate wafers in the shape of circular discs. The diameter of each
wafer is 16 millimeters. What is the area of each candy? So the candy, they say it's
the shape of circular discs. And they tell us that the
diameter of each wafer is 16 millimeters. If I draw a line
across the circle that goes through the
center, the length of that line all the way across
the circle through the center is 16 millimeters. So let me write that. So the diameter here
is 16 millimeters. And they want us to
figure out the area of the surface of this
candy, or essentially, the area of this circle. And so when we
think about area, we know that the area
of a circle is equal to pi times the radius
of the circle squared. And you say, well, they
gave us the diameter. What is the radius? Well, you might remember the
radius is 1/2 of the diameter. It's the distance
from the center of the circle to the outside,
to the boundary of the circle. So it would be
this distance right over here, which is exactly
1/2 of the diameter, so it would be 8 millimeters. So where we see the radius,
we could put 8 millimeters. So the area is going
to be equal to pi times 8 millimeters
squared, which would be 64 square millimeters. And typically, this is
written with pi after the 64. So you might often
see it as this is equal to 64 pi
millimeters squared. Now this is the answer,
64 pi millimeters squared. But sometimes, it's not so
satisfying to just leave it as pi. You might say, well, I want to
get a estimate of what number this is close to. I want a decimal
representation of this. And so, we could start to
use approximate values of pi. So the most rough approximate
value that tends to be used is saying that pi, a
very rough approximation, is equal to 3.14. So in that case, we
could say that this is going to be equal to 64
times 3.14 millimeters squared. And we can get our
calculator to figure out what this will be
in decimal form. So we have 64 times
3.14, gives us 200.96. So we could say that the
area is approximately equal to 200.96
square millimeters. Now if we want to get a
more accurate representation of this-- pi actually
just keeps going on and on and on
forever-- we could use the calculator's
internal representation of pi, in which case,
we'll say 64 times, and then we have to look for
the pi in the calculator. It's up here in
this yellow, so I'll do this little second function. Get the pi there. Every calculator will
be a little different. But 64 times pi. And now we're going to
use the calculator's internal approximation
of pi, which is going to be more precise
than what I had in the last one. And you get 201-- so
let me put it over here so I can write it down--
so more precise is 201. And I'll round to the nearest
hundreds, so you get 201.06. So more precise is 201.06
square millimeters. So this is closer to
the actual answer, because a calculator's
representation is more precise than this very
rough approximation of what pi is.