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Multiplying Rational Expressions

A rational number is any number that can be written as a fraction with integers. An example is 1 2 or 4 15 .

A rational expression is one that can be expressed as a quotient of polynomials. That is, a/b where a and b are polynomials and b 0 .

How to multiply a fraction by a fraction

To multiply a fraction by another fraction, you simply multiply the two numerators and multiply the two denominators and then simplify as needed.

a b × c d = a × c b × d

An example of this would be:

3 7 × 4 5

= 3 × 4 7 × 5

= 12 35

There are no common factors, so this cannot be reduced any further. Therefore, 3 7 × 4 5 = 12 35

How to multiply rational expressions

Multiplying rational expressions is basically the same as multiplying fractions. That is to say, you multiply the numerators to get the numerator of the product and you multiply the denominators to get the denominator of the product.

The rule that guides the multiplication of rational expressions is:

For all rational expressions a b and c d with b 0 and d 0 , a b × c d = a c b d .

The steps to multiply rational expressions are as follows:

  1. Factor all of the polynomials.
  2. Cancel any common factors.
  3. Multiply the resulting numerator and denominator.

Example 1

Multiply the following rational expression.

7 x 2 3 × 9 14 x

Note the variable in the denominator of the second fraction. It cannot be 0 because that would lead us to divide by 0, which is forbidden.

There is no factoring to be done, so our first step is to cancel duplicate factors.

7 x 2 14 x = x 2

9 3 = 3 .

x 1 × 3 2 = 3 x 2

3 x 2 for x

We need to add the x 0 because 3 x 2 makes it possible for x to be 0, but the original expression does not allow for x to be 0 because x is in the second fraction's denominator.

Example 2

Multiply the following rational expression.

2 x + 1 x 2 - 1 × x + 1 2 x 2 + x

First, we see that we can factor each of the denominators. The first denominator is a case of the difference of two squares. The second denominator is easy because we can pull out a factor of x.

2 x + 1 x 2 1 = 2 x + 1 ( x + 1 ) ( x 1 ) × x + 1 x ( 2 x + 1 )

Now we can multiply the numerators and denominators by placing them side by side.

( 2 x + 1 ) ( x + 1 ) ( x + 1 ) ( x - 1 ) ( x ) ( 2 x + 1 )

Now we can factor out common factors. We can factor out 2 x + 1 and x + 1 . That leaves us with

1 ( x - 1 ) ( x )

Which can be simplified to

1 x ( x - 1 )

Topics related to the Multiplying Rational Expressions

Domain and Range of Rational Functions

Rational Equations

Dividing Rational Expressions

Flashcards covering the Multiplying Rational Expressions

Precalculus Flashcards

CLEP Precalculus Flashcards

Practice tests covering the Multiplying Rational Expressions

Precalculus Diagnostic Tests

Get help learning how to multiply rational expressions

Multiplying rational expressions requires a lot of hard work and concentration. If your student struggles with the process, a tutor is an excellent resource to supplement their in-school learning about the topic. A tutor can break down the process into smaller steps and walk your student through each step until they understand each part of the process and how it works toward the whole problem. A 1-on-1 tutor will work at your student's pace, taking extra time to work on especially challenging concepts and skimming through topics that your student picks up easily. This makes tutoring especially efficient in addition to being highly effective. Varsity Tutors can connect your student with a qualified tutor who can meet their individual needs when it comes to learning to multiply rational expressions and other important math topics. Contact us today and speak with one of our Educational Directors to get started.

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