Circumference - Perimeter of circles made easy - fast math lesson. - Free Educational videos for Students in K-12

Circumference - Perimeter of circles made easy - fast math lesson. - By tecmath

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00:0-1	Welcome to take Math Journal . We're going to have
00:02	to look at this video is how to work out
00:04	the perimeter of the circle , which is also known
00:07	as the circumference of a circle . So before we
00:10	start on this and how to work them out ,
00:12	I'm going to just discuss a couple of terms that
00:14	you'll get when you are working with circles . So
00:17	first off we have this idea , the circumference ,
00:20	that is my circle . The circumference is this distance
00:23	Around the outside . Okay , so from one there
00:26	and all the way around is known as the circumference
00:30	. And when we measure circles , there's a couple
00:32	of ways that we can also measure them . We
00:34	can measure them other from this point in the middle
00:39	to the edge . And this here is known is
00:44	the radius or we can use another measurement where what
00:51	we're looking at is say this whole entire distance across
00:58	the circle , but this is known as the dominant
01:05	. What you might realize is that the diameter is
01:07	twice of the radius . Okay , because the radius
01:10	is halfway across , it's in the center to the
01:13	edge . There's another radius , there's a radius ,
01:16	area , radius is the radius is half of the
01:19	diameter . So it's a really really important thing that
01:22	, you know , because we're gonna use the radius
01:24	uh to work out the circumference of the circle .
01:28	And the way we do this is as follows ,
01:30	we're gonna be using we're going to use a formula
01:33	which is there's no way around it . You just
01:35	have to remember a formula to do this and the
01:37	formula resists . So the circumference equals two boy ,
01:43	uh two times pi times on our pie pie is
01:47	the number . Uh It's a it's an irrational number
01:51	. That's basically this number . Which is Is obtained
01:55	by dividing the Circle's circumference by style . And its
01:59	value is around about 3.141592 is by using a calculator
02:12	. Okay , so this is one of these things
02:14	you will be using a calculator for and so I
02:18	put a calculator up , this is the top calculator
02:20	you have on your computer . What about we change
02:22	the view to a scientific one moving across and you
02:27	see that here , Pie ? Okay , so this
02:30	is a calculator that comes with your Computer . Uh
02:33	the other ones that you guys might use will have
02:36	a button that looks like this and that's the way
02:38	you get the most exact answer . So some people
02:41	will go to use 3.14 and I'll get slightly different
02:43	answers . Whatever use pie , the pie button ,
02:46	you'll get the most exact answer . So I'm going
02:48	to work out the circumference of a couple of different
02:51	circles and a couple of different types of questions that
02:54	you might get asked . Okay , I'll leave that
02:57	formula out there . So let's work through it to
03:03	say I have a circle here which is like this
03:09	and I know that the radius is 25 cm .
03:15	So the radius from here to here is 25 centimetres
03:20	and we get asked to know what the circumference is
03:24	, what the perimeter is . So the formula we're
03:26	going to use is circumference equals two times pi times
03:30	up . So This is going to be the circumference
03:33	equals two talent spy times 25 centimeters . Okay ,
03:42	so I'll get the calculated up to work this out
03:46	. So let's work this out now . Uh So
03:49	we end up with two times I times 25 and
03:57	this equals As you'll see here 157.08 to be rounded
04:04	.079 . And then around that nine up to be
04:07	.08 , or 08 So the answer is this equals
04:14	157 0.8 108 . What sentiment ? Okay . So
04:22	how did you go with that ? What about one
04:25	more of these ? So the one I'm going to
04:28	ask you now , I've had that formula up there
04:32	. You say you were given a question that looked
04:34	like this . Maybe we'll actually give you a get
04:37	you to work at the perimeter of this shape here
04:39	. This is a terrible semicircle . Okay , and
04:44	what I'll tell you about it is that this part
04:47	here , The diameter is 20 cm . So how
04:52	did you work out this distance ? Around the outside
04:55	this circumference ? We're going to see . First off
04:58	that this is half of the circumference . There's a
05:01	couple of things we're gonna work out half the circumference
05:04	. All right , so how let's see for this
05:10	perimeter is going to equal half of the circumference as
05:14	well as this story . I'm gonna hear . What's
05:18	this diameter ? Okay , now this diameter Is 20
05:24	, so this means that the radius equals what the
05:26	radius is , half the diamond , so the radius
05:29	is equal to 10 cm . Okay , so now
05:33	I've got all the information that we need to work
05:36	this out . I'll move this over here , add
05:38	we'll use we'll do able to working out over here
05:41	. Okay , so first off let's work out what
05:43	sees Yeah , the circumference all my pen . Again
05:50	, this circumference we're going to work out what half
05:52	it is . So we get the circumference then we'll
05:55	harvard then we'll add it to the diamonds . The
05:57	circumference equals two . So it's pulling Times The Radius
06:03	which is 10 times 10 centimetres . So let's work
06:09	out what that is . Let me get the calculator
06:11	back up so clear that two times hi times 10
06:21	And that equals 62.83 to the Sequels 62 0.83 Central
06:30	americans . Okay , that's our circumference , but we
06:34	want half of the circumference . So half our circumference
06:37	is this amount divided by two Professor , let's just
06:42	get this number here and we'll divide it . What
06:44	suit That equals 31 point 42 Okay , so this
06:52	equals 31 point forward . So this is our half
06:58	, it's a conference then we have to add .
07:02	So we got this measurement here . Okay then in
07:07	fact I can rub this out and put it in
07:09	here now just to show that a little bit better
07:11	this distance from here to here . But going around
07:16	he is 31.42 . I should be putting in the
07:20	units to which is centimeters . Not put them in
07:22	over here as well , centimeters . And so we're
07:25	gonna add these two were doing without so it's fairly
07:29	easy now . Okay , so let's do that .
07:31	I'll rub out the rest of our working so far
07:35	And we're just going to add this to this 20
07:40	, we add these together and we're going to get
07:42	the answer of yes perimeter equals 31.42 centimetres plus 20
07:53	centimeters equals 51.42 centimetres . So how'd you go with
08:03	that ? Pretty good . Hopefully you're pretty good with
08:07	those . That's type of questions that you get with
08:09	these . Okay , so give them a go .
08:11	Um I can put up actually , I put up
08:13	another one right now , just just for the heck
08:15	of it , another type of questions that you might
08:17	get asked with . Why not ? I think the
08:19	more practice we do the better and if you're fine
08:22	with these you might already flick it off and go
08:24	on to the next thing . But I'll go through
08:25	a harder example now if something you may get with
08:28	these , okay to say you were asked to work
08:31	out this particular shape here we had a doorway which
08:36	had a half arch . Okay and then from there
08:42	we have a doorway and it's like the bottom of
08:45	a rectangle there this year is 4cm . This From
08:51	here here is 12 cm but also from here to
08:56	here . So let's work out the perimeter here .
09:01	Okay so we're gonna be using circumference to work all
09:03	this out . So again what we have here is
09:06	a half circumference . Okay so this is uh this
09:09	one is half of the circumference . They're half the
09:15	circumference , so circumference equals two pi r . So
09:20	Ah as you see here is going to be this
09:23	is the diameter which is four . So their radius
09:25	is going to be half of this , which is
09:26	to say this equals two tom boy 22 . Yeah
09:32	. To this equals let's get a calculator so clear
09:37	to times five times two Equals 12.57 , 12 0.57
09:50	This should be in centimeters and this should be in
09:52	sediments . So that's uh that's that would be a
09:56	full circle . We're gonna be half of this .
09:58	So half of C Equals 12.57 divided by two .
10:06	They're called 6.2 equals 6.28 celebrating . Okay , so
10:15	we'll rub this out here and then we can work
10:17	at this perimeter here . So we have this here
10:20	as 6.28 celebrators . So a perimeter is just going
10:27	to be all these together . We have another unknown
10:29	side over here . So let's put him on as
10:31	well . This side is gonna be the same as
10:33	this side . This is going to be equal to
10:34	12cm . So let's work out perimeter . Mhm .
10:45	Primitive equals . We'll start here . 6.28 celebrators plus
10:53	12 plus for plus 12 . This is going to
11:01	equal . Let's have a look uh move this down
11:05	here so we can see what we're doing . Clear
11:08	that 6.28 plus 12 plus four plus 12 For this
11:19	Eagles . 34.2 , That was 34 0.2 . I
11:26	celebrate . That's correct . So how'd you go with
11:30	those ? I hope that was some help . Okay
11:33	, um that's top questions you get with those uh
11:35	circumference ones anyway . See you next time . Yeah
00:0-1	.

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