Multiplying and Dividing Rational Expressions

Learning Outcomes

  • Simplify rational expressions.
  • Multiply and divide rational expressions.

Simplifying Rational Expressions

The quotient of two polynomial expressions is called a rational expression. We can apply the properties of fractions to rational expressions such as simplifying the expressions by canceling common factors from the numerator and the denominator. To do this, we first need to factor both the numerator and denominator. Let’s start with the rational expression shown.

[latex]\frac{{x}^{2}+8x+16}{{x}^{2}+11x+28}[/latex]

[latex]\\[/latex]We can factor the numerator and denominator to rewrite the expression as [latex]\frac{{\left(x+4\right)}^{2}}{\left(x+4\right)\left(x+7\right)}[/latex]

[latex]\\[/latex]
Then we can simplify the expression by canceling the common factor [latex]\left(x+4\right)[/latex] to get [latex]\frac{x+4}{x+7}[/latex]
[latex]\\[/latex]

How To: Given a rational expression, simplify it

  1. Factor the numerator and denominator.
  2. Cancel any common factors.

Example: Simplifying Rational Expressions

Simplify [latex]\frac{{x}^{2}-9}{{x}^{2}+4x+3}[/latex].

Q & A

Can the [latex]{x}^{2}[/latex] term be cancelled in the above example?

No. A factor is an expression that is multiplied by another expression. The [latex]{x}^{2}[/latex] term is not a factor of the numerator or the denominator.

Try It

Simplify [latex]\frac{x - 6}{{x}^{2}-36}[/latex].

Multiplying Rational Expressions

Recall Multiplying fractions

To multiply fractions, multiply the numerators and place them over the product of the denominators.

[latex]\dfrac{a}{b}\cdot\dfrac{c}{d} = \dfrac {ac}{bd}[/latex]

It is helpful to factor the numerator and denominator and cancel common factors before multiplying terms together.

Multiplication of rational expressions works the same way as multiplication of any other fractions. We multiply the numerators to find the numerator of the product, and then multiply the denominators to find the denominator of the product. Before multiplying, it is helpful to factor the numerators and denominators just as we did when simplifying rational expressions. We are often able to simplify the product of rational expressions.

How To: Given two rational expressions, multiply them

  1. Factor the numerator and denominator.
  2. Multiply the numerators.
  3. Multiply the denominators.
  4. Simplify.

Example: Multiplying Rational Expressions

Multiply the rational expressions and show the product in simplest form:

[latex]\frac{{x}^{2}+4x-5}{3x+18}\cdot \frac{2x - 1}{x+5}[/latex]

Try It

Multiply the rational expressions and show the product in simplest form:

[latex]\frac{{x}^{2}+11x+30}{{x}^{2}+5x+6}\cdot \frac{{x}^{2}+7x+12}{{x}^{2}+8x+16}[/latex]

Dividing Rational Expressions

recall dividing fractions

To divide fractions, multiply the first by the reciprocal of the second.

[latex]\dfrac{a}{b}\div\dfrac{c}{d}=\dfrac{a}{b}\cdot\dfrac{d}{c}=\dfrac{ad}{bc}[/latex]

Remember to factor first and cancel common factors in the numerator and denominator before multiplying individual terms together.

Division of rational expressions works the same way as division of other fractions. To divide a rational expression by another rational expression, multiply the first expression by the reciprocal of the second. Using this approach, we would rewrite [latex]\frac{1}{x}\div \frac{{x}^{2}}{3}[/latex] as the product [latex]\frac{1}{x}\cdot \frac{3}{{x}^{2}}[/latex]. Once the division expression has been rewritten as a multiplication expression, we can multiply as we did before.

[latex]\frac{1}{x}\cdot \frac{3}{{x}^{2}}=\frac{3}{{x}^{3}}[/latex]

How To: Given two rational expressions, divide them

  1. Rewrite as the first rational expression multiplied by the reciprocal of the second.
  2. Factor the numerators and denominators.
  3. Multiply the numerators.
  4. Multiply the denominators.
  5. Simplify.

Example: Dividing Rational Expressions

Divide the rational expressions and express the quotient in simplest form:

[latex]\frac{2{x}^{2}+x - 6}{{x}^{2}-1}\div \frac{{x}^{2}-4}{{x}^{2}+2x+1}[/latex]

Try It

Divide the rational expressions and express the quotient in simplest form:

[latex]\frac{9{x}^{2}-16}{3{x}^{2}+17x - 28}\div \frac{3{x}^{2}-2x - 8}{{x}^{2}+5x - 14}[/latex]