Multiplying & Dividing Rational Expressions Lesson

When we multiply or divide rational expressions, we follow the same rules we used with fractions.

Multiplying Rational Expressions

• Factor all numerators and all denominators
• Cancel any common factors other than 1 between the numerators and denominators
• Multiply the remaining factors in the numerators and the remaining factors in the denominators
  • We may choose to leave the rational expression in factored form

Let's look at a few examples.
Example 1: Find each product. $$\frac{7x - 56}{5x^2 + 45x + 40}\cdot \frac{40x^2 + 40x}{8x}$$ Step 1) Factor all numerators and all denominators: $$\frac{7(x - 8)}{5(x + 1)(x + 8)}\cdot \frac{40x(x + 1)}{8x}$$ Step 2) Cancel any common factors other than 1 between the numerators and denominators: $$\require{cancel}\frac{7(x - 8)}{\cancel{5}\cancel{(x + 1)}(x + 8)}\cdot \frac{\cancel{40x}\cancel{(x + 1)}}{\cancel{8x}}$$ Step 3) Multiply the remaining factors in the numerators and the remaining factors in the denominators: $$\frac{7x - 56}{x + 8}$$ It's also valid to report your answer in factored form. $$\frac{7(x - 8)}{x + 8}$$ Example 2: Find each product. $$\frac{-15x^2 - 13x + 72}{15x^2 - 12x - 27}\cdot \frac{3x^2 - 6x - 9}{3x + 8}$$ Step 1) Factor all numerators and all denominators: $$\frac{-1(5x - 9)(3x + 8)}{3(5x - 9)(x + 1)}\cdot \frac{3(x - 3)(x + 1)}{(3x + 8)}$$ Step 2) Cancel any common factors other than 1 between the numerators and denominators: $$\frac{-1\cancel{(5x - 9)}\cancel{(3x + 8)}}{\cancel{3}\cancel{(5x - 9)}\cancel{(x + 1)}}\cdot \frac{\cancel{3}(x - 3)\cancel{(x + 1)}}{\cancel{(3x + 8)}}$$ Step 3) Multiply the remaining factors in the numerators and the remaining factors in the denominators: $$-1(x -3)$$ $$-x + 3$$

Dividing Rational Expressions

When we divide rational expressions, we multiply the first rational expression (leftmost) by the reciprocal of the second (rightmost). Let's look at an example.
Example 3: Find each quotient. $$\frac{15x^2 + 31x + 10}{9x^2 + 30x + 25}÷ \frac{5x^2 - 38x - 16}{3x^2 - 4x - 15}$$ Step 1) Set up the division problem as the multiplication of the first rational expression by the reciprocal of the second: $$\frac{15x^2 + 31x + 10}{9x^2 + 30x + 25}\cdot \frac{3x^2 - 4x - 15}{5x^2 - 38x - 16}$$ Now we can follow our procedure for multiplying rational expressions.
Step 2) Factor all numerators and all denominators: $$\frac{(5x + 2)(3x + 5)}{(3x + 5)(3x + 5)}\cdot \frac{(x - 3)(3x + 5)}{(x - 8)(5x + 2)}$$ Step 3) Cancel any common factors other than 1 between the numerators and denominators: $$\frac{\cancel{(5x + 2)}\cancel{(3x + 5)}}{\cancel{(3x + 5)}\cancel{(3x + 5)}}\cdot \frac{(x - 3)\cancel{(3x + 5)}}{(x - 8)\cancel{(5x + 2)}}$$ Step 4) Multiply the remaining factors in the numerators and the remaining factors in the denominators: $$\frac{x - 3}{x - 8}$$

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