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What number sets does
the number 3.4028
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repeating belong to?
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And before even answering the
question, let's just think
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about what this represents.
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And especially what this
line on top means.
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So this line on top means
that the 28 just
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keep repeating forever.
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So I could express this number
as 3.4028, but the 28 just
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keep repeating.
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Just keep repeating on and
on and on forever.
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I could just keep writing
them forever and ever.
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And obviously, it's just easier
to write this line over
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the 28 to say that it
repeats forever.
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Now let's think about what
number sets it belongs to.
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Well, the broadest number set
we've dealt with so far is the
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real numbers.
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And this definitely belongs
to the real numbers.
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The real numbers is essentially
the entire number
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line that we're used to using.
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And 3.4028 repeating sits
someplace over here.
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If this is negative 1, this
is 0, 1, 2, 3, 4.
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3.4028 is a little bit more
than 3.4, a little
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bit less than 3.41.
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It would sit right over there.
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So it definitely sits
on the number line.
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It's a real number.
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So it definitely is real.
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It definitely is
a real number.
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But the not so obvious question
is whether it is a
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rational number.
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Remember, a rational number is
one that can be expressed as a
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rational expression
or as a fraction.
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If I were to tell you that p is
rational, that means that p
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can be expressed as the
ratio of two integers.
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That means that p can be
expressed as the ratio of two
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integers, m/n.
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So the question is, can I
express this as the ratio of
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two integers?
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Or another way to think
of it, can I
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express this as a fraction?
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And to do that, let's actually
express it as a fraction.
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Let's define x as being
equal to this number.
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So x is equal to 3.4028
repeating.
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Let's think about
what 10,000x is.
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And the only reason why I want
10,000x is because I want to
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move the decimal point all the
way to the right over here.
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So 10,000x.
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What is that going
to be equal to?
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Well every time you multiply
by a power of 10, you shift
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the decimal one to the right.
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10,000 is 10 to the
fourth power.
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So it's like shifting
the decimal over to
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the right four spaces.
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1, 2, 3, 4.
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So it'll be 34,028.
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But these 28's just
keep repeating.
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So you'll still have the 28's
go on and on, and on and on,
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and on after that.
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They just all got shifted to the
left of the decimal point
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by five spaces.
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You can view it that way.
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That makes sense.
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It's nearly 3 and 1/2.
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If you multiply by 10,000,
you get almost 35,000.
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So that's 10,000x.
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Now, let's also think
about 100x.
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And my whole exercise here is
I want to get two numbers
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that, when I subtract them and
they're in terms of x, the
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repeating part disappears.
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And then we can just treat them
as traditional numbers.
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So let's think about
what 100x is.
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100x.
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That moves this decimal point.
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Remember, the decimal point
was here originally.
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It moves it over to the
right two spaces.
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So 100x would be 300-- Let
me write it like this.
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It would be 340.28 repeating.
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We could have put the 28
repeating here, but it
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wouldn't have made
as much sense.
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You always want to write it
after the decimal point.
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So we have to write 28 again to
show that it is repeating.
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Now something interesting
is going on.
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These two numbers, they're
just multiples of x.
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And if I subtract the bottom one
from the top one, what's
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going to happen?
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Well the repeating part
is going to disappear.
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So let's do that.
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Let's do that on both sides
of this equation.
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Let's do it.
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So on the left-hand side of this
equation, 10,000x minus
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100x is going to be 9,900x.
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And on the right-hand side,
let's see-- The decimal part
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will cancel out.
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And we just have to figure out
what 34,028 minus 340 is.
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So let's just figure this out.
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8 is larger than 0, so we
won't have to do any
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regrouping there.
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2 is less than 4.
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So we will have to do some
regrouping, but we can't
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borrow yet because we
have a 0 over there.
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And 0 is less than 3, so we
have to do some regrouping
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there or some borrowing.
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So let's borrow from
the 4 first.
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So if we borrow from the 4, this
becomes a 3 and then this
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becomes a 10.
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And then the 2 can now
borrow from the 10.
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This becomes a 9 and
this becomes a 12.
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And now we can do
the subtraction.
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8 minus 0 is 8.
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12 minus 4 is 8.
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9 minus 3 is 6.
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3 minus nothing is 3.
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3 minus nothing is 3.
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So 9,900x is equal to 33,688.
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We just subtracted 340
from this up here.
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So we get 33,688.
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Now, if we want to solve
for x, we just divide
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both sides by 9,900.
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Divide the left by 9,900.
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Divide the right by 9,900.
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And then, what are
we left with?
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We're left with x is equal
to 33,688 over 9,900.
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Now what's the big
deal about this?
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Well, x was this number. x was
this number that we started
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off with, this number that
just kept on repeating.
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And by doing a little bit of
algebraic manipulation and
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subtracting one multiple of it
from another, we're able to
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express that same exact
x as a fraction.
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Now this isn't in simplest
terms. I mean they're both
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definitely divisible by 2
and it looks like by 4.
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So you could put this in lowest
common form, but we
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don't care about that.
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All we care about is the fact
that we were able to represent
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x, we were able to represent
this number, as a fraction.
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As the ratio of two integers.
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So the number is
also rational.
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It is also rational.
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And this technique we
did, it doesn't only
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apply to this number.
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Any time you have a number that
has repeating digits, you
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could do this.
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So in general, repeating
digits are rational.
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The ones that are irrational are
the ones that never, ever,
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ever repeat, like pi.
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And so the other things, I think
it's pretty obvious,
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this isn't an integer.
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The integers are the
whole numbers that
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we're dealing with.
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So this is someplace in
between the integers.
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It's not a natural number or a
whole number, which depending
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on the context are viewed
as subsets of integers.
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So it's definitely
none of those.
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So it is real and
it is rational.
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That's all we can
say about it.