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Here you will learn strategies on how to add and subtract rational numbers including using visual models, the number line, as well as using standard algorithms.
Students will first learn about integers in 6 th grade math as part of their work with the number system and expand that knowledge to operations with integers in the 7 th grade.
What are adding and subtracting rational numbers?
Adding and subtracting rational numbers is when you add or subtract two or more rational numbers together.
You can add and subtract rational numbers using visual models or a number line.
What are adding and subtracting rational numbers?
Common Core State Standards
How does this apply to 7 th grade math?
Grade 7: The Number System (7.NS.A.1) Apply and extend previous understandings of addition and subtraction to add and subtract rational numbers; represent addition and subtraction on a horizontal or vertical number line diagram.
Grade 7: The Number System (7.NS.A.1a) Describe situations in which opposite quantities combine to make 0. For example, a hydrogen atom has 0 charge because its two constituents are oppositely charged.
Grade 7: The Number System (7.NS.A.1d) Apply properties of operations as strategies to add and subtract rational numbers.
[FREE] Addition and Subtraction Check for Understanding (Grade 2 to 7)
Use this quiz to check your grade 2, 3, 4 and 7 students’ understanding of addition and subtraction. 15+ questions with answers covering a range of 2nd, 3rd, 4th and 7th grade addition and subtraction topics to identify areas of strength and support!
[FREE] Addition and Subtraction Check for Understanding (Grade 2 to 7)
Use this quiz to check your grade 2, 3, 4 and 7 students’ understanding of addition and subtraction. 15+ questions with answers covering a range of 2nd, 3rd, 4th and 7th grade addition and subtraction topics to identify areas of strength and support!
In order to add and subtract rational numbers using counters:
Model the problem with counters and use zero pairs when necessary.
The answer is the leftover counters.
In order to add and subtract rational numbers using a number line:
To add, start at the first number and move to the second number; to subtract, start from the second number and move to the first number.
Write your answer.
Adding and subtracting rational numbers examples
Example 1: adding negative numbers using counters
-2 \, \cfrac{1}{2}+7= \, ?
Model the problem with counters and use zero pairs when necessary.
Use what you know about integer zero pairs.
There is \cfrac{1}{2} of a positive counter and 4 whole positive counters left.
2The answer is the leftover counters.
-2 \, \cfrac{1}{2}+7=4 \cfrac{1}{2}
Example 2: subtracting a negative number using counters
-4.5-(-6)= \, ?
Model the problem with counters and use zero pairs when necessary.
Model -4.5 with the counters.
There are not enough negatives to take away -6. Add 2 zero pairs in order to remove -6.
This leaves 2 positive counters and \cfrac{1}{2} of a negative counter.
There is 1 positive counter left and \cfrac{1}{2} of a positive counter left.
The answer is the leftover counters.
-4.5 - (-6) = 1.5
Example 3: adding decimals with a number line
-11.1+(-0.3)= \, ?
To add, start at the first number and move to the second number; to subtract, start from the second number and move to the first number.
-11.1 + (-0.3) is addition. Start at -11.1 and move in the negative direction (left) 0.3 places. You land at -11.4.
Write your answer.
-11.1 + (-0.3) = -11.4
How to add and subtract rational numbers with algorithms
In order to add and subtract rational numbers using algorithms:
Make sense of the calculation – relating to positive numbers when necessary.
Use an algorithm.
Decide if the final answer is positive or negative.
Example 4: subtracting fractions (mixed numbers) with different denominators
Solve 3 \, \cfrac{4}{5}-5 \, \cfrac{1}{6} \, .
Make sense of the calculation – relating to positive numbers when necessary.
3 \, \cfrac{4}{5}-5 \, \cfrac{1}{6} \, is the distance from 5 \, \cfrac{1}{6} to 3 \, \cfrac{4}{5} \, .
5 \, \cfrac{1}{6}-3 \, \cfrac{4}{5} \, is the distance from 3 \, \cfrac{4}{5} to 5 \, \cfrac{1}{6} \, , which is the exact same part of the number line, just the opposite direction.
This means you can solve 5 \, \cfrac{1}{6}-3 \, \cfrac{4}{5} \, and use the opposite as the answer for 3 \, \cfrac{4}{5}-5 \, \cfrac{1}{6} \, .
Use an algorithm.
5 \, \cfrac{1}{6}-3 \, \cfrac{4}{5}
=\cfrac{31}{6}-\cfrac{19}{5} \hspace{1.4cm} *Convert to improper fractions
=\cfrac{31 \times 5}{6 \times 5}-\cfrac{19 \times 6}{5 \times 6} \quad *Create a common denominator
=\cfrac{155}{30}-\cfrac{114}{30} \hspace{1.15cm} *Subtract the numerators
=\cfrac{41}{30} which simplifies to 1 \, \cfrac{11}{30}
Decide if the final answer is positive or negative.
There are two ways to think about this…
Since for the distance on the number line you start at 5 \, \cfrac{1}{6} and go back to 3 \, \cfrac{4}{5}, the direction is negative.
Since you are taking a larger amount away from a smaller amount, the answer will be negative.
Make sense of the calculation – relating to positive numbers when necessary.
-125.5 + (-34.56) combines two negative amounts, so the answer will be an even larger negative amount. You can combine like you would with positive numbers, to see what the total amount is.
Use an algorithm.
Decide if the final answer is positive or negative.
Adding two negative numbers results in a more negative number. For example, if you owe \$125.50 and then you make another purchase of \$34.56, now you owe even more. Your debt has grown, and debt is negative.
-125.5 + (-34.56) = -160.06
Example 6: adding a positive and negative decimal
Solve 125.5+(-34.56).
Make sense of the calculation – relating to positive numbers when necessary.
125.5 + (-34.56) combines a positive and negative amount. Combining positive and negative creates 0 pairs, which has the same effect as subtracting two positive numbers. You can subtract like we would with positive numbers, to see what the total difference is.
Use an algorithm.
Decide if the final answer is positive or negative.
Now to decide if the answer is positive or negative, look at the original numbers being added. Since 125.5 is larger, the amount left over will also be positive.
125.5 + (-34.56) = 90.94
Teaching tips for adding and subtracting rational numbers
Start with adding and subtracting rational expressions that involve only positive numbers. Students will use these skills to then operate with negative numbers. For students who are struggling to remember all algorithms, it may be necessary to provide them with step-by-step guides.
When choosing worksheets, look for ones that include word problems. These types of problems give the rational numbers a real world context and allow students to practice figuring out which operation is appropriate.
Easy mistakes to make
Getting the positive and negative direction on the number line backwards Right is positive and left is negative.
Confusing the different signs and rules The rules for adding and subtracting integers can be used to solve with rational numbers. However, just like with integers it is important to think about why the rule works when using it, to avoid using the wrong rule. For example,
-5 + 3 = \, ? -5 + (-3) = -8
-5 + 3 = \, ? 5 + 3 = -2
It doesn’t make sense that you would add positive 3 to a -5 and get a more negative number. You should get a less negative number, which is shown in the correct strategy.
Since the original equation started with more negatives, the result will still be negative – although there are less now after subtracting 8 \, \cfrac{1}{2} negatives. Also, moving from -8 \, \cfrac{1}{2} to -14 \, \cfrac{2}{3} is to the left or in the negative direction.
-13.2 + (-4.8) combines two negative numbers, so the sum will be more negative.
You can solve for the sum of the negatives like you would positives since there are no zero pairs canceling values out:
13.2 + 4.8 = 18.
Since the original equation is adding negatives, the answer will also be negative.
-13.2 + (-4.8) = -18
6) On a January day in New York, the morning temperature was -3.5 degrees Fahrenheit. Later that day, the temperature increased by 14.3 degrees. What is the new temperature in degrees?
-10.8 degree
17.8 degrees
-17.8 degrees
10.8 degree
-3.5 increased by 14.3 degrees is -3.5 + 14.3.
Since you can add in any order, you can also solve for 14.3 + (-3.5).
If we start with 14.3 and add -3.5 the answer will go down 3.5, so you can use 14.3-3.5 to solve.
14.3-3.5 = 10.8, so 14.3 + (-3.5) = 10.8 which means -3.5 + 14.3 = 10.8.
Adding and subtracting rational numbers FAQs
How are positive and negative numbers related?
All positive numbers have an inverse that is negative and all negative numbers have an inverse that is positive. For example, -4.5 and 4.5 are inverses. When you add inverses together their sum is always 0 – think zero pairs. Thinking about inverses is one way to solve addition and subtraction of rational numbers.
How do you add fractions with uncommon denominators?
In order to add negative fractions and mixed fractions (mixed numbers), each fraction must have the same denominator. A common strategy is to find the least common multiple (LCM) of the denominators and use this to solve (sometimes called the least common denominator or LCD). Once the denominators are common, you can add the numerators to solve.
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