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In this video we're going
to get introduced to the
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Pythagorean theorem,
which is fun on its own.
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But you'll see as you learn
more and more mathematics it's
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one of those cornerstone
theorems of really all of math.
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It's useful in geometry,
it's kind of the backbone
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of trigonometry.
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You're also going to use
it to calculate distances
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between points.
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So it's a good thing to really
make sure we know well.
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So enough talk on my end.
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Let me tell you what the
Pythagorean theorem is.
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So if we have a triangle, and
the triangle has to be a right
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triangle, which means that one
of the three angles in the
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triangle have to be 90 degrees.
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And you specify that it's
90 degrees by drawing that
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little box right there.
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So that right there is-- let
me do this in a different
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color-- a 90 degree angle.
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Or, we could call
it a right angle.
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And a triangle that has
a right angle in it is
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called a right triangle.
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So this is called
a right triangle.
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Now, with the Pythagorean
theorem, if we know two sides
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of a right triangle we can
always figure out
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the third side.
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And before I show you how to
do that, let me give you one
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more piece of terminology.
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The longest side of a right
triangle is the side opposite
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the 90 degree angle-- or
opposite the right angle.
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So in this case it is
this side right here.
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This is the longest side.
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And the way to figure out where
that right triangle is, and
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kind of it opens into
that longest side.
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That longest side is
called the hypotenuse.
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And it's good to know, because
we'll keep referring to it.
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And just so we always are good
at identifying the hypotenuse,
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let me draw a couple of
more right triangles.
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So let's say I have a triangle
that looks like that.
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Let me draw it a
little bit nicer.
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So let's say I have a triangle
that looks like that.
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And I were to tell you
that this angle right
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here is 90 degrees.
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In this situation this is the
hypotenuse, because it is
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opposite the 90 degree angle.
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It is the longest side.
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Let me do one more, just
so that we're good at
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recognizing the hypotenuse.
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So let's say that that is my
triangle, and this is the 90
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degree angle right there.
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And I think you know how
to do this already.
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You go right what
it opens into.
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That is the hypotenuse.
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That is the longest side.
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So once you have identified the
hypotenuse-- and let's say
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that that has length C.
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And now we're going to
learn what the Pythagorean
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theorem tells us.
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So let's say that C is equal to
the length of the hypotenuse.
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So let's call this
C-- that side is C.
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Let's call this side
right over here A.
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And let's call this
side over here B.
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So the Pythagorean theorem
tells us that A squared-- so
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the length of one of the
shorter sides squared-- plus
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the length of the other shorter
side squared is going to
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be equal to the length of
the hypotenuse squared.
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Now let's do that with an
actual problem, and you'll see
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that it's actually not so bad.
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So let's say that I have a
triangle that looks like this.
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Let me draw it.
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Let's say this is my triangle.
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It looks something like this.
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And let's say that they tell us
that this is the right angle.
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That this length right here--
let me do this in different
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colors-- this length right
here is 3, and that this
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length right here is 4.
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And they want us to figure
out that length right there.
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Now the first thing you want to
do, before you even apply the
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Pythagorean theorem, is to
make sure you have your
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hypotenuse straight.
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You make sure you know
what you're solving for.
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And in this circumstance we're
solving for the hypotenuse.
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And we know that because this
side over here, it is the side
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opposite the right angle.
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If we look at the Pythagorean
theorem, this is C.
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This is the longest side.
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So now we're ready to apply
the Pythagorean theorem.
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It tells us that 4 squared--
one of the shorter sides-- plus
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3 squared-- the square of
another of the shorter sides--
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is going to be equal to this
longer side squared-- the
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hypotenuse squared-- is going
to be equal to C squared.
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And then you just solve for C.
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So 4 squared is the same
thing as 4 times 4.
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That is 16.
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And 3 squared is the same
thing as 3 times 3.
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So that is 9.
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And that is going to be
equal to C squared.
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Now what is 16 plus 9?
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It's 25.
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So 25 is equal to C squared.
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And we could take the positive
square root of both sides.
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I guess, just if you look at
it mathematically, it could
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be negative 5 as well.
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But we're dealing with
distances, so we only care
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about the positive roots.
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So you take the principal
root of both sides and
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you get 5 is equal to C.
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Or, the length of the
longest side is equal to 5.
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Now, you can use the
Pythagorean theorem, if we give
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you two of the sides, to figure
out the third side no matter
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what the third side is.
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So let's do another
one right over here.
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Let's say that our
triangle looks like this.
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And that is our right angle.
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Let's say this side over here
has length 12, and let's say
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that this side over
here has length 6.
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And we want to figure out this
length right over there.
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Now, like I said, the first
thing you want to do is
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identify the hypotenuse.
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And that's going to be the side
opposite the right angle.
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We have the right angle here.
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You go opposite
the right angle.
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The longest side, the
hypotenuse, is right there.
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So if we think about the
Pythagorean theorem-- that A
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squared plus B squared is
equal to C squared-- 12
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you could view as C.
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This is the hypotenuse.
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The C squared is the
hypotenuse squared.
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So you could say
12 is equal to C.
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And then we could say that
these sides, it doesn't matter
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whether you call one of
them A or one of them B.
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So let's just call
this side right here.
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Let's say A is equal to 6.
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And then we say B-- this
colored B-- is equal
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to question mark.
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And now we can apply the
Pythagorean theorem.
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A squared, which is 6 squared,
plus the unknown B squared is
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equal to the hypotenuse
squared-- is equal
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to C squared.
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Is equal to 12 squared.
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And now we can solve for B.
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And notice the difference here.
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Now we're not solving
for the hypotenuse.
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We're solving for one
of the shorter sides.
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In the last example we
solved for the hypotenuse.
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We solved for C.
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So that's why it's always
important to recognize that A
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squared plus B squared plus C
squared, C is the length
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of the hypotenuse.
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So let's just solve for B here.
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So we get 6 squared is 36,
plus B squared, is equal
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to 12 squared-- this
12 times 12-- is 144.
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Now we can subtract 36 from
both sides of this equation.
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Those cancel out.
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On the left-hand side we're
left with just a B squared
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is equal to-- now 144
minus 36 is what?
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144 minus 30 is 114.
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And then you
subtract 6, is 108.
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So this is going to be 108.
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So that's what B squared is,
and now we want to take the
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principal root, or the
positive root, of both sides.
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And you get B is equal
to the square root, the
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principal root, of 108.
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Now let's see if we can
simplify this a little bit.
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The square root of 108.
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And what we could do is
we could take the prime
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factorization of 108
and see how we can
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simplify this radical.
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So 108 is the same thing as 2
times 54, which is the same
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thing as 2 times 27, which is
the same thing as 3 times 9.
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So we have the square root of
108 is the same thing as the
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square root of 2 times 2
times-- well actually,
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I'm not done.
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9 can be factorized
into 3 times 3.
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So it's 2 times 2 times
3 times 3 times 3.
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And so, we have a couple of
perfect squares in here.
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Let me rewrite it a
little bit neater.
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And this is all an exercise in
simplifying radicals that you
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will bump into a lot while
doing the Pythagorean theorem,
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so it doesn't hurt to
do it right here.
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So this is the same thing as
the square root of 2 times 2
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times 3 times 3 times the
square root of that last
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3 right over there.
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And this is the same thing.
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And, you know, you wouldn't
have to do all of
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this on paper.
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You could do it in your head.
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What is this?
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2 times 2 is 4.
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4 times 9, this is 36.
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So this is the square root of
36 times the square root of 3.
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The principal root of 36 is 6.
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So this simplifies to
6 square roots of 3.
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So the length of B, you could
write it as the square root of
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108, or you could say it's
equal to 6 times the
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square root of 3.
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This is 12, this is 6.
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And the square root of 3,
well this is going to be a 1
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point something something.
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So it's going to be a
little bit larger than 6.