Multiply and Divide Rational Expressions
Learning Objectives
- Simplify a rational expression
- Multiply and divide 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 cancelling 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.
How To: Given a rational expression, simplify it.
- Factor the numerator and denominator.
- Cancel any common factors.
Example: Simplifying Rational Expressions
Simplify [latex]\frac{{x}^{2}-9}{{x}^{2}+4x+3}[/latex].Answer: [latex-display]\begin{array}\frac{\left(x+3\right)\left(x - 3\right)}{\left(x+3\right)\left(x+1\right)}\hfill & \hfill & \hfill & \hfill & \text{Factor the numerator and the denominator}.\hfill \\ \frac{x - 3}{x+1}\hfill & \hfill & \hfill & \hfill & \text{Cancel common factor }\left(x+3\right).\hfill \end{array}[/latex-display]
Analysis of the Solution
We can cancel the common factor because any expression divided by itself is equal to 1.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].Answer: [latex]\frac{1}{x+6}[/latex]
Multiplying Rational Expressions
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.
- Factor the numerator and denominator.
- Multiply the numerators.
- Multiply the denominators.
- Simplify.
Example: Multiplying Rational Expressions
Multiply the rational expressions and show the product in simplest form:Answer:
[latex]\begin{array}{cc}\frac{\left(x+5\right)\left(x - 1\right)}{3\left(x+6\right)}\cdot \frac{\left(2x - 1\right)}{\left(x+5\right)}\hfill & \text{Factor the numerator and denominator}.\hfill \\ \frac{\left(x+5\right)\left(x - 1\right)\left(2x - 1\right)}{3\left(x+6\right)\left(x+5\right)}\hfill & \text{Multiply numerators and denominators}.\hfill \\ \frac{\cancel{\left(x+5\right)}\left(x - 1\right)\left(2x - 1\right)}{3\left(x+6\right)\cancel{\left(x+5\right)}}\hfill & \text{Cancel common factors to simplify}.\hfill \\ \frac{\left(x - 1\right)\left(2x - 1\right)}{3\left(x+6\right)}\hfill & \hfill \end{array}[/latex]
Try It
Multiply the rational expressions and show the product in simplest form:Answer: [latex]\frac{\left(x+5\right)\left(x+6\right)}{\left(x+2\right)\left(x+4\right)}[/latex]
Dividing Rational Expressions
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.How To: Given two rational expressions, divide them.
- Rewrite as the first rational expression multiplied by the reciprocal of the second.
- Factor the numerators and denominators.
- Multiply the numerators.
- Multiply the denominators.
- Simplify.
Example: Dividing Rational Expressions
Divide the rational expressions and express the quotient in simplest form:Answer:
[latex]\begin{array}\text{ }\frac{2x^{2}+x-6}{x^{2}}\cdot\frac{x^{2}+2x+1}{x^{2}-4} \hfill& \text{Rewrite as the first rational expression multiplied by the reciprocal of the second rational expression.} \\ \frac{\left(2\times3\right)\cancel{\left(x+2\right)}}{\cancel{\left(x+1\right)}\left(x-1\right)}\cdot\frac{\cancel{\left(x+1\right)}\left(x+1\right)}{\cancel{\left(x+2\right)}\left(x-2\right)} \hfill& \text{Factor and cancel common factors.} \\ \frac{\left(2x+3\right)\left(x+1\right)}{\left(x-1\right)\left(x-2\right)} \hfill& \text{Multiply numerators and denominators.} \\ \frac{2x^{2}+5x+3}{x^{2}-3x+2} \hfill& \text{Simplify.}\end{array}[/latex]
Try It
Divide the rational expressions and express the quotient in simplest form:Answer: [latex]1[/latex]
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