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12.8: Multiplication of Rational Expressions

Difficulty Level: At Grade Created by: CK-12
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What if you had two rational expressions like \begin{align*}\frac{2x^2 - 3}{x - 4}\end{align*}2x23x4 and \begin{align*}\frac{x^2 - 3x + 2}{x^2}\end{align*}x23x+2x2 and you wanted to multiply them? How could you do so such that the answer were in simplest terms? After completig this Concept, you'll be able to multiply rational expressions like this one.

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CK-12 Foundation: 1208S Multiplying Rational Expressions

Guidance

The rules for multiplying and dividing rational expressions are the same as the rules for multiplying and dividing rational numbers, so let’s start by reviewing multiplication and division of fractions. When we multiply two fractions we multiply the numerators and denominators separately:

\begin{align*}\frac{a}{b} \cdot \frac{c}{d}=\frac{a \cdot c}{b \cdot d}\end{align*}abcd=acbd

Multiply Rational Expressions Involving Monomials

Example A

Multiply the following: \begin{align*}\frac{a}{16b^8} \cdot \frac{4b^3}{5a^2}\end{align*}a16b84b35a2.

Solution

Cancel common factors from the numerator and denominator. The common factors are 4, \begin{align*}a\end{align*}a, and \begin{align*}b^3\end{align*}b3. Canceling them out leaves \begin{align*}\frac{1}{4b^5} \cdot \frac{1}{5a} = \frac{1}{20ab^5}\end{align*}14b515a=120ab5.

Example B

Multiply \begin{align*}9x^2 \cdot \frac{4y^2}{21x^4}\end{align*}.

Solution

Rewrite the problem as a product of two fractions: \begin{align*}\frac{9x^2}{1} \cdot \frac{4y^2}{21x^4}\end{align*} Then cancel common factors from the numerator and denominator.

The common factors are 3 and \begin{align*}x^2\end{align*}. Canceling them out leaves \begin{align*}\frac{3}{1} \cdot \frac{4y^2}{7x^2} = \frac{12y^2}{7x^2}\end{align*}.

Multiply Rational Expressions Involving Polynomials

When multiplying rational expressions involving polynomials, first we need to factor all polynomial expressions as much as we can. Then we follow the same procedure as before.

Example C

Multiply \begin{align*}\frac{4x+12}{3x^2} \cdot \frac{x}{x^2-9}\end{align*}.

Solution

Factor all polynomial expressions as much as possible: \begin{align*}\frac{4(x+3)}{3x^2} \cdot \frac{x}{(x+3)(x-3)}\end{align*}

The common factors are \begin{align*}x\end{align*} and \begin{align*}(x + 3)\end{align*}. Canceling them leaves \begin{align*}\frac{4}{3x} \cdot \frac{1}{(x-3)} = \frac{4}{3x(x-3)} = \frac{4}{3x^2-9x}\end{align*}.

Multiply a Rational Expression by a Polynomial

When we multiply a rational expression by a whole number or a polynomial, we can write the whole number (or polynomial) as a fraction with denominator equal to one. We then proceed the same way as in the previous examples.

Example D

Multiply \begin{align*}\frac{3x+18}{4x^2+19x-5} \cdot (x^2+3x-10)\end{align*}.

Solution

Rewrite the expression as a product of fractions: \begin{align*}\frac{3x+18}{4x^2+19x-5} \cdot \frac{x^2+3x-10}{1}\end{align*}

Factor polynomials: \begin{align*}\frac{3(x+6)}{(x+5)(4x-1)} \cdot \frac{(x-2)(x+5)}{1}\end{align*}

The common factor is \begin{align*}(x + 5)\end{align*}. Canceling it leaves \begin{align*}\frac{3(x+6)}{(4x-1)} \cdot \frac{(x-2)}{1} = \frac{3(x+6)(x-2)}{(4x-1)} = \frac{3x^2+12x-36}{4x-1}\end{align*}

Watch this video for help with the Examples above.

CK-12 Foundation: Multiplying Rational Expressions

Vocabulary

  • When we multiply two fractions we multiply the numerators and denominators separately:

\begin{align*}\frac{a}{b} \cdot \frac{c}{d}=\frac{a \cdot c}{b \cdot d}\end{align*}

Guided Practice

Multiply \begin{align*}\frac{12x^2-x-6}{x^2-1} \cdot \frac{x^2+7x+6}{4x^2-27x+18}\end{align*}.

Solution

Factor polynomials: \begin{align*}\frac{(3x+2)(4x-3)}{(x+1)(x-1)} \cdot \frac{(x+1)(x+6)}{(4x-3)(x-6)}\end{align*}.

The common factors are \begin{align*}(x + 1)\end{align*} and \begin{align*}(4x - 3)\end{align*}. Canceling them leaves \begin{align*}\frac{(3x+2)}{(x-1)} \cdot \frac{(x+6)}{(x-6)} = \frac{(3x+2)(x+6)}{(x-1)(x-6)} = \frac{3x^2+20x+12}{x^2-7x+6}\end{align*}

Practice

Multiply the following rational expressions and reduce the answer to lowest terms.

  1. \begin{align*}\frac{x^3}{2y^3} \cdot \frac{2y^2}{x}\end{align*}
  2. \begin{align*}\frac{2x}{y^2} \cdot \frac{4y}{5x}\end{align*}
  3. \begin{align*}2xy \cdot \frac{2y^2}{x^3}\end{align*}
  4. \begin{align*}\frac{4y^2-1}{y^2-9} \cdot \frac{y-3}{2y-1}\end{align*}
  5. \begin{align*}\frac{6ab}{a^2} \cdot \frac{a^3b}{3b^2}\end{align*}
  6. \begin{align*}\frac{33a^2}{-5} \cdot \frac{20}{11a^3}\end{align*}
  7. \begin{align*}\frac{2x^2+2x-24}{x^2+3x} \cdot \frac{x^2+x-6}{x+4}\end{align*}
  8. \begin{align*}\frac{x}{x-5} \cdot \frac{x^2-8x+15}{x^2-3x}\end{align*}
  9. \begin{align*}\frac{5x^2+16x+3}{36x^2-25} \cdot (6x^2+5x)\end{align*}
  10. \begin{align*}\frac{x^2+7x+10}{x^2-9} \cdot \frac{x^2-3x}{3x^2+4x-4}\end{align*}
  11. \begin{align*}\frac{x^2+8x+16}{7x^2+9x+2} \cdot \frac{7x+2}{x^2+4x}\end{align*}

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Vocabulary

TermDefinition
Rational Expression A rational expression is a fraction with polynomials in the numerator and the denominator.

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