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Parts of a circle

Parts of a Circle
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Parts of a circle

Discussion and questions for this video
At 1:58, is the circumfrence like the perimeter?
Yes, it is like the perimeter. Like an angular perimeter.
I wasn't sure of where to put this, but It's good enough. I need help with The Arc Exercise. Can someone help?
Me too. There aren't enough stages in the hints to explain how to do the exercises, and no video.
where is the part that explains the arc of a circle? can you explain?
An arc is just an unbroken portion of a circle's circumference. In other words, if you were to trace around a circle but not go all of the way around (no matter how little or how much), that is an arc.
How do you even spell radius(In plural)?
is it Radiusi or something?
the plural of radius is simply radii
What is the center?
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.
I made a program that shows what Sal did:
https://www.khanacademy.org/cs/circle/1828421139
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!
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."
who made or found the area of a circle
*Archimedes* figured it out a long time ago.
Learn more about him here - http://en.wikipedia.org/wiki/Archimedes
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?
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.
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
I know right! 2 me it dosent make scence
what is the circumference though
The circumference is the distance around a circle. It's like the perimeter, only using a special term.
Thank you so much for your help
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.
What is a radiis
the radis is a line that goes on until the middle of the circle
At 0:49, he says you can have more than one radius. Does this mean that I could have more than one diameter?
yes, you can have more than one diameter.
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
I don't understand how to do Circles and arcs can anyone help explain this to me?
well learnhow to add and subtract your inequality
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!
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
where did pi come from and who worked out pi's formula
my windows down and cruise!
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.
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.
what is the formula for diameter of a circle
the diameter is two times the radius ( a straight line half way across the 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.
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
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
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!
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).
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
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.
where did tau come from?
Here is a video that explains why some people have suggested that Tau is better than pi
https://www.khanacademy.org/math/trigonometry/basic-trigonometry/long_live_tau/v/tau-versus-pi
If you have the circumference of a circle how do you find itś radius.
Please help.
Circumference = 2*pi*radius
Solve for radius, and you see that:
radius = circumference/(2*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?".
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.
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.
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.
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.
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 (:
in a circle what part is like area
The inside of the circumference.
So does radius x 2 = diameter?
yes,
example if the radius of the circle was 3cm...
the diameter would be 3 * 2 = 6cm
how can u find the perimeter??
The perimeter is actually the circumference. It is found by multiplying pi (3.14) times the diameter
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.
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.
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.
Why aren't there other names for triangles? Squares? Why just circles?
Can there be more than one radius?(:
Well, sort of Mr. Steeves. The radius of a circle varies from circle to circle, just as you said. however, a radius can be drawn as a line, but it is really just an integer that can be applied to a diagragm. so actually, although there is only one figure that is constant for each circle, the radius integer could be applied to any degree of the circle, or to be exact, any portion of any degree of a circle. So in reality, there is an infinite number of radii for every circle in the universe. Altho
at 1:08 is the you make the senter line. does that help make this easier?
what can i do to get the area of a 7 circle
who made up the names for the circle
what is the curvature of a circle?
where trying to identify and calculate area and perimeter for each quadrilateral
Is it possible to determine a circle just by an arc?
Could you elaborate on that question a little more? It is a little hard to understand what concept you are trying to figure out.
at 6:42 great video keep doing them they help a lot.
Are these videos in order? Cuz it feels like otherwise
The videos should be in order
if pi is infinate why do we say 3.14?
Pi is an irrational number, which means that it goes on forever and forever (not infinity). We use 3.14 because it is easier and is pretty accurate. You wouldn't want to use 3.14159265358979323... as one of your factors.
should put in tangent and other parts
3.14=pi. so, how do you multiply pi by a million numbers all at one time?
these only 4 parts of a circle?
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.
Does anyone no why the video box is not registering as being watched (in the green) when your done watching? Its really screwing with my ability to know what I have covered when reviewing.
I need help on how to memorize the difference.
How do you measure the circumfrence?
2 π r (two times 3.141 times the radius)
is radius the outside
No, the outside is the circumference. A radius RADIATES out from the center to the edge.
Does the diameter always go straight across?
Yes, the diameter is a going to always be a straight line.
How do you find the measure of the arc?
Is the circumference a synonym for perimeter?
You can think of it that way, but circumference is generally used for circles (as seen here) and perimeter is used for polygons.
is pi the only math symbol that represents a number
Nope. e and i are other popular ones
how many diameters can you put in one circle
There are an infinite amount of diameters, since a diameter is a line that passes through the center of a circle and touches both ends of the circle.
0:00 all the way down to 2:01 hasn't helped me at all.
Try listening to the video again till you understand the video on Parts of a Circle. You could also try listening to the video before the one on Parts of a circle.
what about the arc? The circumference does mean there is a "crescent moon" of the circle?
can you please add a video on circle theorems
thank you!!!
What is dilation of circles
I am having trouble with a question on homework, the question is: The diameter of a sphere s 33 and mAB=54 degrees. Find the distance between A and B. Can someone please help?
so the radius doubled is the length of the diameter
how do you find the measure of the arc
You draw an angle from the endpoints of arc to center of circle, then you find the measure of the angle and that is also the arc measure.
what is the point of tangency
That particular 'point' where the tangent and the curve meet is called the 'Point of Tangency'.
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.
Are there any other videos that can help me with this topic - I DON'T UNDERSTAND!!!
The video, Area of a Circle might help you find out the different parts of a circle.
Is the circumfrence like the perimeter
Yes, the circumference is the perimeter. The perimeter of a circle to be exact.
i just reealized who is the man that is speaking thier?
a line passing through 2 points of a circle!
What is the proof that the tangent of a circle is perpendicular to the diameter that intersects the tangent?
why is circumference named circumference and not perimeter
Latin root is circ meaning round. Circum meaning around. Thus the circumference is the distance "around" an object.
how do i find the circumference if i only have the diameter
to find a circumference u just multiply the diameter and pie which is 3.14 or 22/7
C=PI*D
do you have to memorize the parts of a circle?
Thank you Very Helpful. So Circumfrence is the whole circle?
no its the perimeter of the whole circle. The whole circle would be like area.
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.
how do i dilate this circle
is a diameter always striaght
Yes, the diameter is always straight and is a 180 degree central angle 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?
at 2:08 ,Is the radius half of the diameter cause it looks like that
yes it goes from the side of the circle to the center called the center
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.
Radius ,what is the formula for it?
r=circumference/2pi or r^2=area/pi=>r=root of area/pi!
i thought that a circle could n't be measured until i found ou a lil piece at the top is flat.But is their another way to maesure a circle?
is circumferance like perimeter? ( 1:58)
Yes, at 1:58, he is referencing the circumference, which is just another word for the perimeter of the circle.
where can i get videos on arc properties and tangent properties???
can u help me with how 2 find the length of the diagonals inside a circle