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## Algebra (all content)

### Unit 13: Lesson 6

Solving rational equations- Rational equations intro
- Rational equations intro
- Equations with one rational expression (advanced)
- Rational equations (advanced)
- Equations with rational expressions
- Equations with rational expressions (example 2)
- Rational equations
- Equation with two rational expressions (old example)
- Equation with two rational expressions (old example 2)
- Equation with two rational expressions (old example 3)

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# Equation with two rational expressions (old example 2)

Sal solves the equation 5/(2x)-4/(3x)=7/18 by first finding the LCM (least common multiple) of 2x and 3x. Created by Sal Khan and Monterey Institute for Technology and Education.

## Video transcript

Solve the equation, 5 over
2x minus 4 over 3x is equal to 7 over 18. And they tell us that x can't to
be equal to 0, because that would make these two expressions
here undefined. Hopefully the answer here is not
0, and then this becomes-- this is kind of extra,
unnecessary information. So let's figure out
how to solve this. So a good place to start-- I
don't like having x's in my denominators. So let's multiply-- and in fact,
in general, I don't like having fractions in
my equations. So let's see if we can multiply
both sides of this equation by some things that
will get rid of the fractions. So let me just rewrite it
so we have some space. 5 over 2x, and then we have
minus 4 over 3x is equal to 7 over 18. Now, if we want to get rid of
the 2 in the denominator here, we could multiply
everything by 2. If we want to get rid of this 3
in the denominator, we could multiply everything by 3. If we want to get rid of this
18 in the denominator, we could multiply everything
by 18. And 18 also includes
a 2 and a 3. The prime factorization
of 18 is 2 times 9, which is 3 times 3. So when you're multiplying both
sides of the equation by 18, you're actually multiplying
it by a 2 and a 3 and another 3. So let's just multiply both
sides of this equation by 18. So I'll multiply this term
right here by 18. And then, this term
right here by 18. That'll get rid of all of
these numbers in the denominator. 18 divided by 3 is 6. 18 divided by 2 is 9. But we don't just have numbers
in the denominator, we also have these x's in
the denominator. So let's also multiply both
sides of the equation by x, so that we get rid of these. So we're essentially going to
multiply both sides of the equation by 18x. We're taking, essentially,
the least common multiple of 2x, 3x and 18. This is the smallest number that
is divisible by all three of these characters. Now, when we do that our
denominators will disappear. x divided by x is 1. 18 divided by 2 is 9. So this term becomes 9
times 5, which is 45. And then this term right here,
x divided by x is 1. 18 divided by 3 is 6. So you have 6 times 4 is 24,
but you have a subtraction sign here so minus 24 is equal
to-- let me do that in that yellow-- is equal to-- and then
you have this term, 7 over 18 times 18x. Well, the 18's cancel out and
you're just left with 7 times x is equal to 7x. And now this becomes a much,
much simpler equation. Now, what's 45 minus 24? Let's see, 45 minus 20
would be would be 25. Then you subtract
4 more, it's 21. So you get 21 is equal to 7x. Divide both sides by 7 and
you get x is equal to 3. And let's verify that
that works. So we have 5 over 2x. So that's the same thing as
5 over 2 times 3, minus 4 over 3 times 3. So this is 5/6 minus 4 over
18-- sorry, 4 over 9. We want to find a common
denominator. 18 is the least common multiple
of 6 and 9, so let's put it over 18. 5/6 is the same thing
as 15 over 18. Multiply the numerator
and denominator by 3. 4/9 is the same thing
as 8 over 18. Multiply the numerator
and denominator by 2. 15 minus 8. So this becomes 15 minus
8 over 18, which is equal to 7 over 18. So it works out. 5 over 2x when x is equal to 3,
minus 4 over 3x when x is equal to 3 is indeed
equal to 7/18. So we're done.