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## Multiplying & dividing rational expressions

# Dividing rational expressions

CCSS.Math: ,

## Video transcript

Divide and express as a
simplified rational. State the domain. We start off with
this expression. We actually have one rational
expression divided by another rational expression. And like we've seen multiple
times before, these rational expressions aren't defined when
their denominators are equal to 0. So p plus 5 cannot be equal to
0, or if we subtract both sides of this-- we can't call
it an equation, but we could call it a not-equation-- by
negative 5, if we subtract negative 5 from both sides, you
get p cannot be equal to-- these cancel out-- negative 5. That's what that tells us. Over here, we could do the same
exercise 4p plus 20 also cannot be equal to 0. If it was, this expression
would be undefined. Subtract 20 from both sides. 4p cannot be equal
to negative 20. Divide both sides by 4. p cannot be equal
to negative 5. So in both situations, p being
equal to negative 5 would make either of these rational
expressions undefined. So the domain here is the set
of all reals such-- or p is equal to the set of all reals
such that p does not equal negative 5, or essentially all
numbers except for negative 5, all real numbers. We've stated the domain, so now
let's actually simplify this expression. When you divide by a fraction or
a rational expression, it's the same thing as multiplying
by the inverse. Let me just rewrite this
thing over here. 2p plus 6 over p plus 5 divided
by 10 over 4p plus 20 is the same thing as multiplying
by the reciprocal here, multiplying by
4p plus 20 over 10. I changed the division into a
multiplication and I flipped this guy right here. Now, this is going to be equal
to 2p plus 6 times 4p plus 20 in the numerator. I won't skip too many steps. Let me just write that. 2p plus 6 times 4p plus 20 in
the numerator and then p plus 5 times 10 in the denominator. Now, in order to see if we can
simplify this, we need to completely factor all of the
terms in the numerator and the denominator. In the numerator, 2p plus 6, we
can factor out a 2, so the 2p plus 6 we can rewrite
it as 2 times p plus 3. Then the 4p plus 20, we
can rewrite that. We can factor out a 4 as--
so 4 times p plus 5. Then we have our p plus 5 down
there in the denominator. We have this p plus 5. We can just write it down
in the denominator. Even 10, we can factor that
further into its prime components or into its
prime factorization. We can factor 10
into 2 times 5. That's the same thing as 10. Let's see what we
can simplify. Of course, this whole time, we
have to add the caveat that p cannot equal negative 5. We have to add this restriction
on the domain in order for it to be the same
expression as the one we started off with. Now, what can we cancel out? We have a 2 divided by a 2. Those cancel out. We have a p plus 5 divided
by a p plus 5. We know that p plus 5 isn't
going to be equal to 0 because of this constraint, so we
can cancel those out. What are we left with? In the numerator, we have 4
times p plus 3, and in the denominator, all we have is that
green 5, and we're done! We could right this as 4/5 times
p plus 3, or just the way we did it right there. But we don't want to forget
that we have to add the constraint p cannot be equal
to negative 5, so that this thing is mathematically
equivalent to this thing right here.