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Welcome to my math blog! The purpose of this blog is to help you stay informed about our learning and experiences that have taken place during our math class. I have also included links your child (and you) may want to use in order to supplement math learning in 5th grade.

Monday, September 19, 2016

Using Models to Add and Subtract Fractions with Unlike Denominators

Lesson Frame

  • We will represent and solve addition and subtraction of fractions with unequal denominators.
  • I will use fraction strips to model addition and subtraction of fractions with unequal denominators.

Whole Class Lesson

I began by having my students add a fraction strip diagram to their math journal.

I demonstrated how to use the fraction strip to find equivalent fractions using a ruler.  We lined up the ruler with 1/2 and determined that 2/4, 3/5, 4/8, 5/10, and 6/12 were all equivalent.  We repeated this with 1/4, 2/3, 3/5, etc.  

Once we had become comfortable with finding equivalents (more or less), we started adding and subtracting fractions with unlike denominators.

I have them the problem 1/2 + 1/4
  • First I demonstrated how to use the ruler to find the fractions that were equal to 1/2.  We rediscovered that 1/2 shares a common denominator with 1/4s.
  • We determined that we would change 1/2 to 2/4 (equivalent fraction) and then we could add.
We continued this process using our fraction strip table to help us find common denominators.  Then we used our common denominator to help us create equivalent fractions.  Finally, we were able to add or subtract.

When we located a problem that required us to change both fractions in order to find a common denominator, I exposed them to the "butterfly" method.

Image result for butterfly fraction method

We did discover that using the butterfly method could mean that we need to simplify our fraction.  

Image result for divisibility rules

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