Section: 不同分母的表达式求值 | 6.9不同分母的表达式求值

Section outline

Leslie is combining the leftover cookies from the bake sale. One pan had $\begin{aligned} \frac{1}{4} \end{aligned}$ of a dozen left. Another pan had $\begin{aligned} \frac{1}{3} \end{aligned}$ of a dozen left. Just then, a teacher came by and bought a half dozen cookies. How many cookies were left?
In this concept, you will learn how to evaluate numerical expressions involving fractions with different denominators.
Evaluating Expressions with Different Denominators
To evaluate an expression that involves adding and subtracting fractions with like denominators, follow the order of operations. Resolve the operations with parenthesis first, and then add or subtract the fractions going from left to right. The process is similar when evaluating an expression with different denominators. Before evaluating the expression, rewrite the fractions with the lowest common denominator.
Here is an expression of fractions.
$\begin{aligned} \frac{1}{2} + \frac{1}{3} - \frac{1}{6} = \underline{} \end{aligned}$
The denominators are 2, 3, and 6. Find the lowest common denominator by finding the lowest common multiple (LCM) of 2, 3, and 6.
2: 2, 4, 6, 8, 10, 12, . . .
3: 3, 6, 9, 12, . . .
6: 6, 12, . . .
The lowest common multiple for all three denominators is 6. Find the equivalent fractions of $\begin{aligned} \frac{1}{2} \end{aligned}$ and $\begin{aligned} \frac{1}{3} \end{aligned}$ with the denominator 6. The fraction $\begin{aligned} \frac{1}{6} \end{aligned}$ is already in terms of sixths.
$\begin{aligned} \frac{1}{2} & = \frac{3}{6} \\ \frac{1}{3} & = \frac{2}{6} \end{aligned}$
Rewrite the expression with the equivalent fractions.
$\begin{aligned} \frac{1}{2} + \frac{1}{3} - \frac{1}{6} = \frac{3}{6} + \frac{2}{6} - \frac{1}{6} \end{aligned}$
Evaluate the expression in order from left to right. Add or subtract the numerators and write the result over the common denominator.

$\begin{aligned} \frac{3}{6} + \frac{2}{6} = \frac{5}{6} - \frac{1}{6} = \frac{4}{6} \end{aligned}$
Finally, write the fraction in simplest form. The greatest common factor (GCF) of 4 and 6 is 2. Divide the numerator and denominator by 2.

$\begin{aligned} \frac{4 \div 2}{6 \div 2} = \frac{2}{3} \end{aligned}$
The answer is $\begin{aligned} \frac{2}{3} \end{aligned}$ .
Examples
Example 1
Earlier, you were given a problem about Leslie and the cookies.
Leslie combine one-fourth and one-third of a dozen cookies, and then sold a half dozen cookies to a teacher. Add and subtract the fractions to find out how many cookies were left.
First, write an expression to solve the problem.
$\begin{aligned} \frac{1}{4} + \frac{1}{3} - \frac{1}{2} \end{aligned}$
Then, rewrite the fractions using the lowest common denominator. The LCD is 12.
$\begin{aligned} \frac{1}{4} + \frac{1}{3} - \frac{1}{2} = \frac{3}{12} + \frac{4}{12} - \frac{6}{12} \end{aligned}$
Next, add or subtract in order from left to right.
$\begin{aligned} \frac{3}{12} + \frac{4}{12} = \frac{7}{12} - \frac{6}{12} = \frac{1}{12} \end{aligned}$
There was $\begin{aligned} \frac{1}{12} \end{aligned}$ of a dozen cookies left. A dozen is equal to 12. Therefore, there was 1 cookie left.
Example 2
Evaluate the expression: $\begin{aligned} \frac{6}{9} + \frac{1}{3} - \frac{4}{5} \end{aligned}$ . Answer in simplest form.
First, find the lowest common denominator using the lowest common multiple of 9, 3, and 5. The LCD is 45.
Then, rewrite the fractions with the common denominator.
$\begin{aligned} \frac{6}{9} & = \frac{30}{45} \\ \frac{1}{3} & = \frac{15}{45} \\ \frac{4}{5} & = \frac{36}{45} \end{aligned}$
$\begin{aligned} \frac{6}{9} + \frac{1}{3} - \frac{4}{5} = \frac{30}{45} + \frac{15}{45} - \frac{36}{45} \end{aligned}$
Next, add or subtract in order from left to right.
$\begin{aligned} \frac{30}{45} + \frac{15}{45} = \frac{45}{45} - \frac{36}{45} = \frac{9}{45} \end{aligned}$
Finally, simplify the fraction.
$\begin{aligned} \frac{9}{45} = \frac{1}{5} \end{aligned}$
The answer is $\begin{aligned} \frac{1}{5} \end{aligned}$ .
Example 3
Evaluate the expression: $\begin{aligned} \frac{4}{5} + \frac{2}{10} - \frac{1}{2} \end{aligned}$ . Answer in simplest form.
First, find the LCD of 5, 10, and 2. The LCD is 10.
Then, rewrite the fractions with the common denominator.
$\begin{aligned} \frac{4}{5} + \frac{2}{10} - \frac{1}{2} = \frac{8}{10} + \frac{2}{10} - \frac{5}{10} \end{aligned}$
Next, add or subtract in order from left to right.
$\begin{aligned} \frac{8}{10} + \frac{2}{10} = \frac{10}{10} - \frac{5}{10} = \frac{5}{10} \end{aligned}$
Finally, simplify the fraction.
$\begin{aligned} \frac{5}{10} = \frac{1}{2} \end{aligned}$
The answer is $\begin{aligned} \frac{1}{2} \end{aligned}$ .
Example 4
Evaluate the expression: $\begin{aligned} \frac{4}{8} - \frac{1}{2} + \frac{1}{4} \end{aligned}$ . Answer in simplest form.
First, find the LCD of 8, 2, and 4. The LCD is 8.
Then, rewrite the fractions with the common denominator.
$\begin{aligned} \frac{4}{8} - \frac{1}{2} + \frac{1}{4} = \frac{4}{8} - \frac{4}{8} + \frac{2}{8} \end{aligned}$
Next, add or subtract in order from left to right.
$\begin{aligned} \frac{4}{8} - \frac{4}{8} = \frac{0}{8} + \frac{2}{8} = \frac{2}{8} \end{aligned}$
Finally, simplify the fraction.
$\begin{aligned} \frac{2}{8} = \frac{1}{4} \end{aligned}$
The answer is $\begin{aligned} \frac{1}{4} \end{aligned}$ .
Example 5
Evaluate the expression: $\begin{aligned} \frac{3}{4} - \frac{5}{8} + \frac{1}{4} \end{aligned}$ . Answer in simplest form.
First, find the LCD of 4, 8, and 4. The LCD is 8.
Then, rewrite the fractions with the common denominator.
$\begin{aligned} \frac{3}{4} - \frac{5}{8} + \frac{1}{4} = \frac{6}{8} - \frac{5}{8} + \frac{2}{8} \end{aligned}$
Next, add or subtract in order from left to right.
$\begin{aligned} \frac{6}{8} - \frac{5}{8} = \frac{1}{8} + \frac{2}{8} = \frac{3}{8} \end{aligned}$
The fraction is in simplest form.
The answer is $\begin{aligned} \frac{3}{8} \end{aligned}$ .
Review
Evaluate the following expressions. Answer in simplest form.
$\begin{aligned} \frac{1}{2} + \frac{1}{3} + \frac{2}{4} \end{aligned}$
$\begin{aligned} \frac{6}{9} + \frac{1}{3} - \frac{2}{3} \end{aligned}$
$\begin{aligned} \frac{4}{5} + \frac{1}{3} - \frac{1}{5} \end{aligned}$
$\begin{aligned} \frac{8}{9} + \frac{1}{2} - \frac{1}{3} \end{aligned}$
$\begin{aligned} \frac{3}{4} + \frac{1}{3} - \frac{2}{10} \end{aligned}$
$\begin{aligned} \frac{3}{4} + \frac{1}{3} + \frac{1}{2} \end{aligned}$
$\begin{aligned} \frac{1}{5} + \frac{2}{5} - \frac{2}{7} \end{aligned}$
$\begin{aligned} \frac{5}{6} + \frac{1}{3} - \frac{1}{2} \end{aligned}$
$\begin{aligned} \frac{8}{9} + \frac{1}{3} - \frac{2}{9} \end{aligned}$
$\begin{aligned} \frac{8}{11} + \frac{1}{3} - \frac{2}{3} \end{aligned}$
$\begin{aligned} \frac{6}{7} + \frac{1}{2} - \frac{2}{7} \end{aligned}$
$\begin{aligned} \frac{4}{9} + \frac{2}{9} - \frac{2}{3} \end{aligned}$
$\begin{aligned} \frac{11}{12} + \frac{1}{12} - \frac{6}{8} \end{aligned}$
$\begin{aligned} \frac{13}{14} + \frac{1}{28} - \frac{4}{7} \end{aligned}$
$\begin{aligned} \frac{17}{18} + \frac{2}{18} - \frac{5}{9} \end{aligned}$
Review (Answers)
Click HERE to see the answer key or go to the Table of Contents and click on the Answer Key under the 'Other Versions' option.
Resources

Section outline

Evaluating Expressions with Different Denominators

Examples

Example 1

Example 2

Example 3

Example 4

Example 5

Review

Review (Answers)

Resources