Perimeter, area, and volume

# Circumference and area of circles

## Labeling parts of a circle

Radius, diameter, center, and circumference--all are parts of a circle. Let's go through each and make sure we understand how they are defined.
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## Labeling parts of a circle

Discussion and questions for this video
How do you even spell radius(In plural)?
How many radiis are in a circle.
EG,

The radius (plural: radii) of a circle is a line from the center of the circle to any point on the circle. So in one sense, you could say there are an infinite number of radii, because there are an infinite number of lines you could draw from the center of the circle to some point on the circle.

But in another sense, each circle has 1 radius… because although you could draw that line an infinite number of ways, EVERY single one of them would have the same length. So people typically say things like, "that circle has a radius of 3 units," or "this circle's radius is 7 centimeters."
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What is the center?
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The center of a circle is the point at which the distance to any part on the circle is exactly the same distance. The center is at the middle of the diameter and bisects the diameter to form two equivalent radii.
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I made a program that shows what Sal did:
The center is also called the origin. The formula A=πr² means that the area of a circle is equal to its radius times the radius times pi. The other formula C=2rπ means that a circle's circumference is equal to two radius (or diameter) times pi.
Hope this helps, if you don't understand!
thanks send it to me!
who made or found the area of a circle
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*Archimedes* figured it out a long time ago.
Is pi equal to 22/7?
Pi is irrational, and so it cannot be written as a fraction. The decimal itself never ends. 22/7 is slightly larger than what pi is, but you can use it for the purpose of calculations.
are diameters of the same circle equal if radii of the same circle are equal?
Yes. The diameter is 2 times the radius. So, for example, if the two radii were 1 then their diameter would be 2 for both of them. You can also think of it this way if r=r then 2r=2r.
the center on a soccer field has a diameter of 20 yards. what is the circumference of the circle?
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maybe you have malfunctioning tech
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what is the formula for diameter of a circle
the diameter is two times the radius ( a straight line half way across the circle).
how do you find circumfrance
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you can find the circumference by multiplying 2 by Pi by the radius.
the radis is a line that goes on until the middle of the circle
what is the circumference though
The circumference is the distance around a circle. It's like the perimeter, only using a special term.
Hello can some one help me I was doing the practice problem and it said I'm close to the right answer but I don't know how to put the right answer can some one help me at least show me how to type it in. Here's what it said Your answer is close, but you may have approximated pi. Enter your answer as a multiple of pi, like 12 pi or 2/3 pi
What did you answer? If you are multiplying, say, X, by pi, write Xpi. You can also always click on the "Answer Format" button on top of the answer bar to see how your answer should look.
are there parts of a oval or is it impossible to specify an oval?
Stick with the geometry videos and you'll find out pretty soon!
where did pi come from and who worked out pi's formula
I think that the number pi (3.14159... and so on) was made by the latin, but I am not completly sure, the only reason i think that is because, the symbol that is used for pi is a latin letter/symbol.
Are there other types of lines with specific names (like diameter and radius) other than a radius and diameter? Are there any with specific names that do not go through the center?
If there was a line across a circle that didn't go through the center, we wouldn't need to give it a name or measure it, because it wouldn't give us any useful information regarding the circle.
For instance, if the line just went across a small part near the edge of the circle, we could measure it, but we wouldn't be able to do anything with the information-- i.e., find out the area of the circle.
We can only use lines that go through or come from the center of the circle to get information about the circle. Therefor, other measures are irrelevant.
Hope this helped! :)
At 0:49, he says you can have more than one radius. Does this mean that I could have more than one diameter?
You can have more than one diameter but they are all the same length.
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Thank you so much for your help
if a protracror has a specified set of pionts ( 108) could you set two of them above another to make a circle you weould have 360 specified pionts but dosnt a circle have an infinite set of points
You need to clarify "point" and "degree." A point is one of the undefined terms of Geometry. It just means a place or spot. It doesn't even have any dimension - it's just there! But that helps us build up some kind of understanding of all the shapes around us.

The markings on the protractor, on the other hand, are degrees. It's a specific way of measuring how far apart two segments are at the vertex where they meet. We also use degrees to talk about how far around a circle you might travel in connection with a central angle (any angle whose vertex is the center of the circle).

So circles have both things going for them, but they're two different things: there are an infinite set of points or spots going around a circle because a point can be any size...or really, has no size at all (which is why there's an infinite number of them); and there's 360 degrees in rotating all the way around a circle.
I am confused why do we use pi in this equation
I think that's because almost all equations (if not all) relating to circles use pi
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I need help and now
How many radii does a circle have?
How many diameters does a circle have?
A circle has an infinite number of radii and diameters since any line drawn through its center is its diameter and half of that line is the radius.
So, there can be multiple radius (or radii or whatever) and diameters but there's only one center right?
There are an infinite amount of lines protruding from the center, so therefore there are infinite radii and diameters. And because only one place is the exact middle of the circle, you are right, there is only one center.
In 1:03 can a radius come from outside of the circle also?
A radius is a line from the center of a circle to the circumference.
Is there a part of a circle called the chord? Did Sal mention it?
A line that touches two points on a circle is a chord.
How come sometimes when you're trying out to find the diameter of something, cause I know that diameter = 2 times the radius, say for example, you're trying to find the diameter of 14pi, you would take half of that and originally multiply it by 2, but the answer would only result in half the number (7)? Can someone explain this to me?
The diameter is two times the radius and the circumference is two times the radius times pi, so since the diameter is two times the radius, the diameter is simply the circumference divided by pi, which in your case results with 14 instead of 7(you found the radius).
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why dose he always repete things
sal just wants to make sure he gets his point across. some people need to hear things more than once
I know that pi is approximately 3.141592653589793238462643383279, but what are tau and e? What are their approximate values? And how and where do we use these irrational numbers?
(for example, pi is very useful when dealing with the area and circumference of circles)
the core is the middle of earth so from earth crust 2 core thats radius ####Smart (:
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How do you type PI into the answer on the challenges?
You just type it in lowercase letters: pi

BTW, you can type π if you are using a Windows PC by typing Alt 227 (press and hold [Alt}, type on the numberpad (_not the main keys_) 227, then release [Alt] ).

However, I don't think the challenge software accepts π, but I am not certain.
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So does radius x 2 = diameter?
yes,
example if the radius of the circle was 3cm...
the diameter would be 3 * 2 = 6cm
what about the diamiter? I dont understand how you go from a raidius to a diamiter and back
The radius of a circle is always half of the diameter. If you are given that the radius is 10, you immediately know that the diameter is 20. And if you are given the diameter, just divide it by 2 to get the radius- for example, if you are given that the diameter is 20, simply diving this by 2 tells you that the radius is 10.

Does that make sense?
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If you have the circumference of a circle how do you find itś radius.
Solve for radius, and you see that:
If you have the area of a cricle how do you find it's circumference.
the are is r squared so if its 49 the radius is 7 and with that info you can find the circumference
if you want to graph a circle, would that require knowing calculus?
Not at all. You can graph a circle using geometry. The center-radius form of a circle is
(x-h)^2 + (y-k)^2 = r^2
Center = (h, k)
I don't understand how to do Circles and arcs can anyone help explain this to me?
how can u find the perimeter??
The perimeter is actually the circumference. It is found by multiplying pi (3.14) times the diameter
what is the difference between the Perimeter and area?
Perimeter is the distance around the edge. Area is the space bounded by the perimeter.
Example:
If you had a square plot of land that measured 3ft on each side, then the perimeter would be 3 + 3 + 3 + 3 = 4 x 3 = 12 ft. The area would be 3 x 3 = 9 sq ft.
What is a circumference? I do it in my course one class in sixth grade, and I know how to do it, but I don't know what circumference means. Please comment or answer my question!
Circumference is the distance around a circle.
at 1:58, is the circumfrence like the perimeter
The circumference is the same concept as the perimeter, but make sure to not mix them up.
If you have the location of the center of a circle and the radius length it is easy to draw a circle.
However, if you have a circle with its center not marked, and you don't know and/or cannot measure the circumference of the circle, how would you go about locating the center ?
I suppose if you bisected the circle (as you would a line segment, using a compass) twice you could locate the exact centrepoint, however I don't think this situation you described would realistically occur.
where did tau come from?
Here is a video that explains why some people have suggested that Tau is better than pi
this was areally confusing and hard to follow, is there an easier video to explain this kind of struff
Sorry if this isn't the answer you were looking for, but the video basically says that:

1) A circle shows all the points at a certain distance from a certain point. This certain point is called the *Center of the circle*. This certain distance is called *Radius*.

2) Doubling the radius gives the *Diameter* (which is basically two radii put together).

3) The *Circumference* is the length of the circle. You could say that it helps you to measure how large the circle is i.e a bigger circle has a larger circumference. If you imagine that the circle is formed by a string, the length of the string is the circumference.

It is actually quite simple, and I would recommend viewing the video a couple of times if it isn't immediately obvious. If you have any further queries, you can always ask a more 'to the point' question like "What is [something]" or "What does Sal say in the video at XX:XX?".
pi actually came from ancient India under the rule of the gupta king Chandhra Gupta II during India's golden age
That's interesting , how did they come up with that.
So the diameter the 2 times the radius or the radius squared?
The radius is a line that goes from the center to the edge. The diameter is a line that goes across the circle, through the center. So the diameter has to be 2 times the radius.
can the radius be anywhere? i know it has to be half of the diameter. so where the diameter is, half of that is the radius?
That didn't really answer my question though. yes to the 1st one or second one? Thank-you!
What is the difference between a sector and a chord of a circle ?
A chord is a line segment that connects two points of a circle. A sector is like a pie slice that is made up of two radii and the arc between them.
1:10
so there are a infinate number of raidus?
yes, a circle has infinite radii, as you can keep on changing the angle by a small amount
My question is that I was told that pi is equal to 3.14 and 22/7 is that true or not?
No, neither is true. π is an irrational and transcendental number. You cannot express it exactly by any algebraic fraction or decimal.
So, 3.14 or 22/7 are *very* rough approximations of π.
If you want better approximations, 3.1415927 should cover most needs. For a fraction I suggest 355/113 which is accurate to 7 digits.
How is knowing how to find out the circumference and radius of a circle help me in life?
We use circles a lot in our society. If you work in manufacturing, it is important for tires, gears, belts, pulleys, etc. Circular geometry is the basis of much of our trigonometry, (Have you heard of the "unit circle"? You will!) which is used in basic physics, architecture, road design, and many other fields.
but even if pi is a series of 9 numbers put together in an order that doesn't repeat, shouldn't it have an end of the combinations? :/
like, 3.14159... and it's in a specific order, and it doesn't repeat. shouldn't it be ending somehow?
Good question. If you include 0, pi is actually made up of combinations of 10 numbers put together. The number of possible combinations of 10 digits is 10^10 = 10 billion. Now that's a lot!

But does pi end or stop being irrational after 10 times 10 billion digits? The answer is no. In reality, the fact that there are 10 billion possible combinations of 10 digits has nothing to do with the properties of pi. When mathematicians say pi has absolutely no repetition in its decimal sequence, they really mean it.

The combinations of digits don't repeat themselves in any sort of pattern. In other words pi is NOT periodic. Pi is an irrational number and that means that it can't be represented as a fraction and there are literally an infinite number number of digits in non-repeating sequences after its decimal point. A periodic number would be something like 0.123412341234... , 1.67516751675..., or even a number with all 10 billion combinations of 10 digits repeating themselves infinitely would be considered periodic. Such numbers all have repeating sequences of digits, but pi doesn't. It may appear to to be periodic, but it really isn't because each apparent "pattern" you might notice in pi is totally random and it's followed by another random combination of digits unrelated to the placement of other random combinations of digits within an infinite sequence of digits.

Here's an overview of proofs that pi is irrational that may be worth looking at: http://en.wikipedia.org/wiki/Proof_that_%CF%80_is_irrational
Warning: The math is rather advanced.
in a circle what part is like area
The inside of the circumference.
where can i get videos on arc properties and tangent properties???
Who made the words perimeter and area?
Perimeter is of Greek origin. Peri means "around", and meter means "measure".
Area is just from the Latin word area, which means "open space".

How many numbers are in pi?
Pi is an irrational number, and it doesn't have a repetition pattern or finite number of digits. I'm not sure, but I think the most digits mathematicians calculated of pi is something around 205 million digits.
do you have to memorize the parts of a circle?
Hi, I'm having a problem proving that a circle is identical/ lies on a circle,
Let's say I have 4 points. A being (-3,1) B (2,4) C (5,3) D=(6,2) with center of (2,-1)
Any ideas?
i know what all these parts of the circle are, but i would like if you could help me understand chord, secant, and tangent in relation to circles? do u have a video for that? im in 10th grade geometry an really struggling. plz help!
Try the search box for each of these terms. I've definitely seen videos on each of these. They may not all be described with respect to circles, as they also fall in other areas.
If pi is a never ending number with infinite decimal places how did they prove it was never ending in the first place?
Pi is infinite, but eventually over time it repeats itself. Therefore, making it stop at one point but keep going..... If that makes any sense..
I have heard the term, but what is a chord?
A chord is a line segment within a circle that does not have to go through the center of the circle. Diameters are the longest chords in a circle because they go through the center. Chords become shorter as they get farther from the center of the circle.
Is cercumfrene perimeter for circles?
yes,the circumference is the perimeter of the circle wait nevermind some one already awnsered
this gets kinda confusing but thx for your help
.
Try to understand the concept and practice it until u get it and then u advance yourself to next step.
is the circumfrance like the diameter ?
Sure, circumference is like the diameter, except that it is about 3.14 times as long as the diameter, and it is curved into a circle shape. Circumference is the distance around a circle. Diameter is the distance across.
What is a secant?
a line passing through 2 points of a circle!
thank you!!!
People say there is something about a chord. What is a chord and what is it's place in a circle?
A chord is a line segment joining any two points in a circle. Diameter of a circle is the longest chord of any circle.
how do you change from radiant to degrees step bye step how do you know when to use it?
180 degrees is pi radians. So how much is 90 degrees? Easy, right?
How can you get an arc out of a circle? And how would you find the arc in the circle in terms of pi.
At 0:07 did you said the circle?
What is the proof that the tangent of a circle is perpendicular to the diameter that intersects the tangent?
Why aren't there other names for triangles? Squares? Why just circles?
how do i find the circumference of a circle
Circumference is equal to 2 * pi * the radius of the circle, or pi * diameter since 2* radius = diameter.
I'm not sure what Pi is I've heard of it and I know It's the Circumference of a circle. But how do I use it?
Pi is describes the relationship between the radius of a circle and its circumference. If you divide the circumference of a circle by twice its radius, you will ALWAYS get Pi. That is, Pi = circumference/(2*Pi). Re-arranging this equation, you get the familiar formula from school, circumference = 2*pi*r.

This video is kind of nice: