Division of Rational Expressions
Just as with ordinary fractions, we first rewrite the division problem as a multiplication problem and then proceed with the multiplication as outlined in the previous section.
Note: Remember that ab÷cd=ab⋅dc. The first fraction remains the same and you take the reciprocal of the second fraction. Do not fall into the common trap of flipping the first fraction.
First convert into a multiplication problem by flipping the second fraction and then simplify as usual:
Dividing a Rational Expression by a Polynomial
When we divide 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, and then proceed the same way as in the previous examples.
Rewrite the expression as a division of fractions, and then convert into a multiplication problem by taking the reciprocal of the divisor:
Then factor and solve:
Solve Applications Involving Multiplication and Division of Rational Expressions
Suppose Marciel is training for a running race. Marciel’s speed (in miles per hour) of his training run each morning is given by the function x3−9x, where x is the number of bowls of cereal he had for breakfast. Marciel’s training distance (in miles), if he eats x bowls of cereal, is 3x2−9x. What is the function for Marciel’s time, and how long does it take Marciel to do his training run if he eats five bowls of cereal on Tuesday morning?
time=distancespeedtime=3x2−9xx3−9x=3x(x−3)x(x2−9)=3x(x−3)x(x+3)(x−3)time=3x+3If x=5, thentime=35+3=38
Marciel will run for 38 of an hour.
Divide the rational functions and reduce the answer to lowest terms.
- Maria’s recipe asks for 212 times more flour than sugar. How many cups of flour should she mix in if she uses 313 cups of sugar?
- George drives from San Diego to Los Angeles. On the return trip he increases his driving speed by 15 miles per hour. In terms of his initial speed, by what factor is the driving time decreased on the return trip?
- Ohm’s Law states that in an electrical circuit I=VRc. The total resistance for resistors placed in parallel is given by: 1Rtot=1R1+1R2. Write the formula for the electric current in terms of the component resistances: R1 and R2.
To view the Review answers, open this PDF file and look for section 12.9.