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For many topics in mathematics teaching the concept before the algorithm can lead to deeper learning. Teaching addition and subtraction of fractions with different denominators is one such topic. Using pattern blocks for this topic gives students a visual representation that they can translate to the algorithm for this topic.
You may want to review equivalent fractions before proceeding. If you read my article on using pattern blocks to show equivalent fractions, you will have a reference point for this lesson.
Remind students that the hexagon represents 1, the trapezoid ½, the rhombus ⅓, and the triangle 1/6. Students can begin adding and subtracting fractions with like denominators using these blocks. Be sure that students make the connection that they are working with blocks of the same color for each of these problems.
Once students have tried a few problems like this you can move onto adding fractions with unlike denominators. Present the class with the following problem.
Discuss with students how this problem is different from the earlier ones. Be sure they realize that these two fractions can’t be added in their current form. Students should see if there are any pattern blocks that are exactly equal to both the trapezoid and rhombus. Once students realize that the triangles work for both the trapezoid and rhombus have them set the pattern blocks as seen below.
Lead them through the transition from the pattern blocks to the original problem.
After students complete a few more problems like this you can introduce the traditional algorithm for this topic. I do recommend leaving the pattern blocks available for those who need them. From my experience they will give up concrete objects on their own when they are comfortable with the process.
Here are a few websites that you might want to view about this topic. I am also including the link for virtual pattern blocks in case you find those to be more beneficial for your students: