Reading: Multiplying Fractions
Learning Objective(s)
·Multiply two or more fractions.
·Multiply a fraction by a whole number.
·Multiply two or more mixed numbers.
·Solve application problems that require multiplication of fractions or mixed numbers.
Introduction
Just as you add, subtract, multiply, and divide when working with whole numbers, you also use these operations when working with fractions. There are many times when it is necessary to multiply fractions and mixed numbers. For example, this recipe will make 4 crumb piecrusts:
5 cups graham crackers 8 T. sugar
cups melted butter 
tsp. vanilla
Suppose you only want to make 2 crumb piecrusts. You can multiply all the ingredients by 
, since only half of the number of piecrusts are needed. After learning how to multiply a fraction by another fraction, a whole number or a mixed number, you should be able to calculate the ingredients needed for 2 piecrusts.
When you multiply a fraction by a fraction, you are finding a "fraction of a fraction.” Suppose you have 
of a candy bar and you want to find of the :
By dividing each fourth in half, you can divide the candy bar into eighths.
Then, choose half of those to get 
.
In both of the above cases, to find the answer, you can multiply the numerators together and the denominators together.
Multiplying Two Fractions
Example:
Multiplying More Than Two Fractions
Example:
|
Example | ||
Problem |
| Multiply. |
| Multiply the numerators and multiply the denominators. | |
| Simplify, if possible. This fraction is already in lowest terms. | |
Answer |
| |
If the resulting product needs to be simplified to lowest terms, divide the numerator and denominator by common factors.
Example | ||
Problem |
| Multiply. Simplify the answer. |
| Multiply the numerators and multiply the denominators. | |
| Simplify, if possible. | |
| Simplify by dividing the numerator and denominator by the common factor 2. | |
Answer |
| |
You can also simplify the problem before multiplying, by dividing common factors.
Example | ||
Problem |
| Multiply. Simplify the answer. |
| Reorder the numerators so that you can see a fraction that has a common factor. Simplify.
| |
Answer |
| |
You do not have to use the "simplify first” shortcut, but it could make your work easier because it keeps the numbers in the numerator and denominator smaller while you are working with them.
A) B) C) D) |
When working with both fractions and whole numbers, it is useful to write the whole number as an improper fraction (a fraction where the numerator is greater than or equal to the denominator). All whole numbers can be written with a "1” in the denominator. For example: 
, 
, and 
. Remember that the denominator tells how many parts there are in one whole, and the numerator tells how many parts you have.
Multiplying a Fraction and a Whole Number
Example:
|
Often when multiplying a whole number and a fraction the resulting product will be an improper fraction. It is often desirable to write improper fractions as a mixed number for the final answer. You can simplify the fraction before or after rewriting as a mixed number. See the examples below.
Example | ||
Problem |
| Multiply. Simplify the answer and write as a mixed number. |
| Rewrite 7 as the improper fraction | |
| Multiply the numerators and multiply the denominators. | |
| Rewrite as a mixed number. | |
Answer |
| |
.
Example | ||
Problem |
| Multiply. Simplify the answer and write as a mixed number. |
| Rewrite 4 as the improper fraction | |
| Multiply the numerators and multiply the denominators. | |
| Simplify. | |
Answer |
| |
.
A) B) C) D) |
If you want to multiply two mixed numbers, or a fraction and a mixed number, you can again rewrite any mixed number as an improper fraction.
So, to multiply two mixed numbers, rewrite each as an improper fraction and then multiply as usual. Multiply numerators and multiply denominators and simplify. And, as before, when simplifying, if the answer comes out as an improper fraction, then convert the answer to a mixed number.
Example | ||
|
| Multiply. Simplify the answer and write as a mixed number. |
| Change | |
| Change | |
| Rewrite the multiplication problem, using the improper fractions. | |
| Multiply numerators and multiply denominators. | |
| Write as a mixed number.
| |
Answer |
| |
to an improper fraction. 5 • 2 + 1 = 11, and the denominator is 5.
to an improper fraction. 2 • 4 + 1 = 9, and the denominator is 2.
Example | ||
Problem |
| Multiply. Simplify the answer and write as a mixed number. |
| Change | |
| Rewrite the multiplication problem, using the improper fraction in place of the mixed number. | |
| Multiply numerators and multiply denominators. | |
| Rewrite as a mixed number.
| |
| Simplify the fractional part to lowest terms by dividing the numerator and denominator by the common factor 2. | |
Answer |
| |
to an improper fraction. 3 • 3 + 1 = 10, and the denominator is 3.
As you saw earlier, sometimes it's helpful to look for common factors in the numerator and denominator before you simplify the products.
Example | ||
Problem |
| Multiply. Simplify the answer and write as a mixed number. |
| Change | |
| Change | |
| Rewrite the multiplication problem using the improper fractions. Reorder the numerators so that you can see a fraction that has a common factor. Simplify. | |
| Multiply. | |
| Write as a mixed fraction. | |
Answer |
| |
to an improper fraction. 5 • 1 + 3 = 8, and the denominator is 5.
to an improper fraction. 4 • 2 + 1 = 9, and the denominator is 4.
In the last example, the same answer would be found if you multiplied numerators and multiplied denominators without removing the common factor. However, you would get 
, and then you would need to simplify more to get your final answer.
A) B) C) D) |
Now that you know how to multiply a fraction by another fraction, by a whole number, or by a mixed number, you can use this knowledge to solve problems that involve multiplication and fractional amounts. For example, you can now calculate the ingredients needed for the 2 crumb piecrusts.
Example | ||
Problem | 5 cups graham crackers 8 T. sugar
| The recipe at left makes 4 piecrusts. Find the ingredients needed to make only 2 piecrusts. |
Since the recipe is for 4 piecrusts, you can multiply each of the ingredients by | ||
| 5 cups graham crackers: Since the result is an improper fraction, rewrite | |
4 T. sugar is needed. | 8 T. sugar: This is another example of a whole number multiplied by a fraction. | |
|
| |
|
| |
Answer | The ingredients needed for 2 pie crusts are: 4 T. sugar
| |
: 2 • 1 + 1, and the denominator is 2. Then, rewrite the multiplication problem, using the improper fraction in place of the mixed number. Multiply.
Often, a problem indicates that multiplication by a fraction is needed by using phrases like "half of,” "a third of,” or "
of.”
Example | ||
Problem | The cost of a vacation is $4,500 and you are required to pay | |
| You need to find | |
| Change 4,500 to an improper fraction by rewriting it with 1 as the denominator. | |
| Divide. | |
900 | Simplify. | |
Answer | You will need to pay $900 when you reserve the trip. | |
of that amount when you reserve the trip. How much will you have to pay when you reserve the trip?
of 4,500. "Of” tells you to multiply.
Example | ||
Problem |
| The pie chart at left represents the fractional part of daily activities. Given a 24-hour day, how many hours are spent sleeping? Attending school? Eating? Use the pie chart to determine your answers. |
| Sleeping is | |
| Rewrite 24 as an improper fraction with a denominator of 1. | |
8 hours sleeping | Multiply numerators and multiply denominators. Simplify | |
| Attending school is | |
| Rewrite 24 as an improper fraction with a denominator of 1. | |
4 hours attending school | Multiply numerators and multiply denominators. Simplify | |
| Eating is | |
| Rewrite 24 as an improper fraction with a denominator of 1. | |
2 hours spent eating | Multiply numerators and multiply denominators. Simplify | |
Answer | Hours spent: sleeping: 8 hours attending school: 4 hours eating: 2 hours | |
of the pie, so the number of hours spent sleeping is 
to 8.
to 4.
of the pie, so the number of hours spent eating is 
to 2.Neil bought a dozen (12) eggs. He used A) 8 B)4 C)9 D)3 |
Summary
You multiply two fractions by multiplying the numerators and multiplying the denominators. Often the resulting product will not be in lowest terms, so you must also simplify. If one or both fractions are whole numbers or mixed numbers, first rewrite each as an improper fraction. Then multiply as usual, and simplify.
Permissions
This reading is taken from the Developmental Math Open Program created by The NROC Project. It is available under a Creative Commons license.
最后修改: 2016年08月25日 星期四 09:58