2.3: Subtraction of Rational Numbers
Learning Objectives
At the end of this lesson, students will be able to:
 Find additive inverses.
 Subtract rational numbers.
 Evaluate change using a variable expression.
 Solve real world problems using fractions.
Vocabulary
Terms introduced in this lesson:
 additive inverse
 opposite
 opposite/inverse process
 subtraction
 method of prime factors
 simplify
 simplest form
 invisible denominator
 speed
 difference
 change
Teaching Strategies and Tips
General Tip: Additive inverses provide a way to subtract two real numbers. Students learn that the operation of subtraction can be replaced with addition by adding the additive inverse of the number.
Subtraction by adding additive inverses applies as well to fractions.
Example:
Although it may be easier for students to keep it as subtraction:
Tips on factor trees:
 Do not include the trivial factors of
1 in the branches of factor trees.  The first two branches of the factor tree can be any pair of factors.
 For instance, in Example 2,
90=9×10 and126=9×14 . Students may choose differently:90=45×2 and126=21×6 .  To find the LCM of three numbers, no more than three factor trees need to be constructed. In general, there will be as many factor trees as numbers whose LCM is being sought. See Example 4 for three numbers.
In Example 4, the “invisible denominator” is a useful way of getting students to realize that
That is, integers are rational numbers. Every whole number can be viewed as a rational number whose denominator is
Use Examples 5 and 6 to demonstrate the concept of change.
 A variable expression is used to evaluate change in speed and change in light intensity, respectively. The functions are provided already; and therefore do not need to be derived from scratch.
 The focus is on the concept of change, which will be new for many students. Exploration is encouraged through tables and graphs.
 To find the change in a quantity using a function, cast this as a subtraction problem: subtract first quantity from second quantity.
 Explaining the results in words will benefit students. Positive change in speed means an increase in speed. The change is negative in Example 6: the intensity reduces as the train travels farther away from the light source.
Error Troubleshooting
General Tip: For many students, the combination of subtraction and finding common denominators is prone to errors. Practice is crucial. See Examples 3 and 4 as well as Review Questions 1a – 1i.
Example 5 and 6:

Change=Speed 2−Speed 1 
Change=Intensity 2−Intensity 1
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Date Created:
Feb 22, 2012Last Modified:
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