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Find the difference.
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Express the answer as a
simplified rational
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expression, and state
the domain.
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We have two rational
expressions, and we're
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subtracting one from
the other.
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Just like when we first learned
to subtract fractions,
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or add fractions, we have to
find a common denominator.
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The best way to find a common
denominator, if were just
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dealing with regular numbers, or
with algebraic expressions,
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is to factor them out, and
make sure that our common
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denominator has all of the
factors in it-- that'll ensure
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that it's divisible by the
two denominators here.
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This guy right here is
completely factored-- he's
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just a plus 2.
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This one over here, let's see if
we can factor it: a squared
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plus 4a plus 4.
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Well, you see the pattern that
4 is 2 squared, 4 is 2 times
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2, so a squared plus 4a plus 4
is a plus 2 times a plus 2, or
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a plus 2 squared.
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We could say it's a plus 2 times
a plus 2-- that's what a
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squared plus 4a plus 4 is.
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This is obviously divisible
by itself-- everything is
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divisible by itself, except, I
guess, for 0, is divisible by
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itself, and it's also divisible
by a plus 2, so this
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is the least common multiple of
this expression, and that
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expression, and it could be
a good common denominator.
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Let's set that up.
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This will be the same thing as
being equal to this first term
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right here, a minus 2 over
a plus 2, but we want the
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denominator now to be a plus 2
times a plus 2-- we wanted it
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to be a plus 2 squared.
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So, let's multiply this
numerator and denominator by a
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plus 2, so its denominator is
the same thing as this.
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Let's multiply both the
numerator and the denominator
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by a plus 2.
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We're going to assume that a
is not equal to negative 2,
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that would have made this
undefined, and it would have
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also made this undefined.
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Throughout this whole thing,
we're going to assume that a
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cannot be equal to negative 2.
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The domain is all real numbers,
a can be any real
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number except for negative 2.
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So, the first term is that--
extend the line a little bit--
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and then the second term doesn't
change, because its
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denominator is already the
common denominator.
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Minus a minus 3 over-- and we
could write it either as a
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plus 2 times a plus 2, or
as this thing over here.
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Let's write it in the factored
form, because it'll make it
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easier to simplify later on:
a plus 2 times a plus 2.
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And now, before we-- let's set
this up like this-- now,
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before we add the numerators,
it'll probably be a good idea
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to multiply this out right
there, but let me write the
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denominator, we know what
that is: it is a plus 2
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times a plus 2.
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Now this numerator: if we have
a minus 2 times a plus 2,
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we've seen that pattern
before.
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We can multiply it out if you
like, but we've seen it enough
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hopefully to recognize that this
is going to be a squared
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minus 2 squared.
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This is going to be
a squared minus 4.
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You can multiply it out, and the
middle terms cancel out--
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the negative 2 times a cancels
out the a times 2, and you're
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just left with a squared minus
4-- that's that over there.
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And then you have this: you have
minus a minus 3, so let's
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be very careful here-- you're
subtracting a minus 3, so you
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want to distribute the negative
sign, or multiply
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both of these terms
times negative 1.
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So you could put a minus a here,
and then negative 3 is
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plus 3, so what does
this simplify to?
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You have a squared minus a
plus-- let's see, negative 4
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plus 3 is negative 1, all
of that over a plus 2
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times a plus 2.
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We could write that as
a plus 2 squared.
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Now, we might want to factor
this numerator out more, to
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just make sure it doesn't
contain a common factor with
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the denominator.
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The denominator is just 2a
plus 2 is multiplied by
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themselves.
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And you can see from inspection
a plus 2 will not
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be a factor in this top
expression-- if it was, this
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number right here would be
divisible by 2, it's not
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divisible by 2.
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So, a plus 2 is not one of the
factors here, so there's not
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going to be any more
simplification, even if we
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were able to factor this thing,
and the numerator out.
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So
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we're done.
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We have simplified the rational
expression, and the
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domain is for all a's, except
for a cannot, or, all a's
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given that a does not equal
negative 2-- all a's except
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for negative 2.
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And we are done.