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So we've got a circle here--
doesn't look like a perfect
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circle, but we can use our
imaginations --and let's say
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it's got a radius of 3 meters.
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My question, or the question
we're going to answer in this
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video is what is the
area of this circle?
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And remember, the area is just
how much space this circle
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takes up on a surface, or on
this computer screen that
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you're watching, or on
this piece of paper.
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If this was a room, it's how
much carpeting you would need
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to fill out this circular room.
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That's what the area is.
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Now, I'm not going to prove it
to you, and we'll do that
-
later, but the area for circle
just takes on a fairly
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straightforward formula and I
want to just get you used to
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applying that formula.
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So the area of a circle
is equal to pi.
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Remember, pi was that number
that people figured out was the
-
ratio between the circumference
and the diameter of the circle.
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It's 3.14159, keeps
going on and on and on.
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It's just the number, but
it's a very magical number.
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Pi times the radius squared.
-
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In fact another way of defining
pi:-- you could even rewrite
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this right here --the area over
your radius squared- so
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this is your radius.
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If you multiply the radius
times itself you could imagine
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that would be the area of a
cube that's like that --that
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the ratio between the area of
this entire circle and the
-
ratio of this cube right
here-- or this square.
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I shouldn't say a cube.
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Cube would be if we went into
3D --but the ratio of the area
-
of the circle to this square
right here is also
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equal to pie.
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That could be actually
an alternate way of
-
defining what pi is.
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And if you were to measure it
very carefully using-- there's
-
thousands of methods you could
do it --you would get 3.14159
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and keep going on
and on and on.
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But we're not going to delve
too deeply into that.
-
Maybe one day I'll make a
whole play list on pi.
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But we just need to know that
the area is equal to pi times
-
r squared, so let's just
apply the numbers here.
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So in our example, the area is
equal to pi times 3 meters
-
squared, which is equal to pi
times 9 meters squared, or the
-
conventional way to write this
is, equal to 9pi
-
meters squared.
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And remembered 9pi, the
convention is just to leave it
-
that way, but this is the same
thing as 9 times 3.14159, which
-
is probably going to be, like,
28 point something
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meters squared.
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Just remember, this is just
some number and it's not 9.
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It's actually closer to 28
because it's going to be
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9 times 3.14159, but we
just leave it like that.
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And that normally will be
good enough for you to
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say, hey that is my area.
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That's my area: 9pi.
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Now let's go the other way:
let's say I have a circle and
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let's say that someone would
say that the area
-
is equal to 16pi.
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What is the diameter of
that circle going to be?
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Well, we know that area
is equal to pi times
-
the radius squared.
-
So at least let's
figure out the radius.
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So the area, 16pi, is equal to
pi times our radius squared.
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I'm just applying this formula.
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We're just going to keep
applying this formula
-
over and over again when
we're dealing with area.
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So area, which we've been
told is 16pi, is equal to
-
pi times radius squared.
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Now, if we divide both sides of
this equation by pi, we get
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16 is equal to r squared.
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And then you take the square
root of both sides and
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you get 4 is equal to r.
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I guess r could also be equal
to negative 4, but we're
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dealing with distances here;
you can't have a
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negative radius.
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Or at least in the world
we're living in right now.
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Just keep things simple;
we just want to keep our
-
distances positives.
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So let's say that this
has a radius of 4.
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Now if the radius is 4,
what is its diameter?
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Well, the diameter is always
going to be 2 times the radius.
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So this 4, we're going to
have another 4 there.
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The diameter is equal to 8.
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Now let's do a slightly harder
one that will kind of compound
-
some of the other things that
we've learned in the past.
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So let's say that I
have a circle here.
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Let's say that its
circumference is equal to 20pi,
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and I want to know its area.
-
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So way you do all these
problems is just figure out
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everything you can, given what
they've given you, and then
-
maybe you can work out the
thing they're asking for.
-
So if I know that the
circumference is 25, what do
-
I know about its radius?
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Well, we saw in the last video
that the circumference is equal
-
to 2pi times the radius.
-
So if the circumference is
equal to 20pi, we could write
-
that 20pi is the circumference
is equal to 2pi
-
times the radius.
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Now if you divide both sides of
this by pi, those cancel out.
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Then if you divide both sides
by 2, this becomes a 1, this
-
becomes a 10, or you get
the radius is equal to 10.
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Which makes sense, right?
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2pi times 10 is going
to be equal to 20pi.
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So we've figured
out our radius.
-
Now, we know that the area is
equal to pi times r squared.
-
And lucky for us, using the
circumference, we were able
-
to figure out the radius.
-
Now using the radius, we
can figure out the area.
-
So the area is going to be
equal to pi times r squared,--
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r is 10 --times 10 squared,
which is equal to pi times 100.
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Or it's equal to 100pi.
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Just like that.
-
so your circumference was 20pi,
when you went around the
-
circle, but your area of
your circle is 100pi.
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And if I gave you units it
would be 100pi units squared.
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That is your area
right there: 100pi.
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Anyway, I think that's pretty
good initial exposure to
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the area of a circle.
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I'll see in the next video.