Subtracting Fractions MathBitsNotebook.com Topical Outline | Jr Math Outline | MathBits' Teacher Resources Terms of Use   Contact Person: Donna Roberts

Rules for Subtracting fractions: 1. The bottom numbers (denominators) must be the same. 2. Subtract the top numbers (numerators)     Put this answer over the denominator. 3. Simplify the fraction, if possible. Remember: "common denominators" ("common denominators" means the "same" denominators) Same Denominators: "Common Denominators" When the fractions have the same denominators, adding and subtracting are easy. When adding (or subtracting) fractions with the same denominators, just add (or subtract) the numerators. Since the denominators are the same, the answer can be found by subtracting the numerators (tops). This answer can be simplified (reduced), since both 3 and 9 can be divided by 3. The "cross lines" in the 4th block means subtract it. Using the "separate parts" method we saw in adding fractions:       Notice the parentheses in the first line. You must subtract the ENTIRE amount of the second mixed number. Without the parentheses, you would be ADDING 5/11. When the parentheses are removed in the next step, you can see 5/11 being subtracted. (The subtraction has been distributed across the parentheses.) Sometimes, during subtraction, you need to borrow, so you will have a sufficient amount from which to subtract. Consider the following: We just borrowed a "1" from 7, changed it to "5/5" and created a fraction with a larger numerator. A fraction with a larger numerator may be useful when subtrating (coming next). Using the "line up" method: You can immediately see a problem: you cannot subtract 4/5 from 2/5. We need to borrow a "1" or "5/5" from the 7, and add that amount to the 2/5.     Visualize: The red squares are being subtracted. Different Denominators: When fractions have different denominators, you must work with common denominators. When adding (or subtracting) fractions with different denominators, you must find a common denominator before adding (or subtracting). Least common denominator of 6 and 9 is 18.         Least common denominator of 4 and 6 is 12. Borrowing will be needed.        This type of problem can be confusing. Think carefully before writing your final answer. First, let's LOOK at what the answer should be with a visual model. Take away the brown squares. The answer should be .   "Separate parts horizontally method": For this question, this method can be a bit confusing. Be careful! "Line up vertically" method: This method gives you a bit of a hint that something is missing in this problem. Be careful not to simply "drop" the 2/3 down into the answer. You could think of the empty oval as . We need to "borrow" 1 from the 4:

Under the Addition of Fractions, we saw how placing the fractions on number lines necessitated determining an equal number of subdivisions for both lines. That same approach applies when working with subtraction.

On the "thirds" line, if we cut each "third" into two equal sections we will get 6 in total.
On the "halves" line, if we cut each "half" into three equal sections we will get 6 in total.
(This number 6 will be called our common denominator.)

Subtract:

Now that both lines contain subdivisions of the same size (both have 6 sections), we can express our fractions using this new subdivision size of 1/6. Then we can subtract the fractions.

Let's combine our number lines onto one line to demonstrate this difference.

For help with fractions on your calculator, click here.