Familiarisation task: Subtracting fractions with unlike denominators

Familiarisation task: Subtracting fractions with unlike denominators

Carry out this familiarisation task to understand how Fractions Lab will help you to subtract fractions with unlike denominators. Click here to download this worksheet.

The Task

Make 3/4 and 2/3 using the same type of representation. Extract the numerators by double clicking on them. Try to subtract them by dragging them together and clicking 'subtract'. Watch the animation. Why do you think Fractions Lab hasn't given you the answer this time? What do you need to do to 3/4 and 2/3 so that Fractions Lab will subtract them? Use the 'find equivalent' tool to change 3/4 and 2/3 into fractions that Fractions Lab will subtract. (You can do this using 'find equivalent' until the two fractions have the same denominator). What fractions did you need to make? Now add the two new fractions together. Drag the three fractions into the 'add/subtract box'. Check the 'subtract' sign is clicked.

Teachers' Notes

This task takes students through several steps to subtract fractions with unlike denominators. You may want to demonstrate these steps and ask students to follow them and to complete the accompanying worksheet that encourages them to reflect on their work.

It is worth noting that Fractions Lab will only add or subtract fractions that have like denominators. This is an intentional design decision to encourage students to consider the importance of identifying common denominators (i.e., identifying equivalent fractions where the denominators are the same) when they later move on to adding and subtracting fractions with unlike denominators. Therefore this task helps students to change the fractions into fractions with like denominators using the 'find equivalent' tool.

You could discuss with students how changing the fractions using the 'find equivalent' tool helps them to think about finding fractions with like denominators in a way that supports their conceptual understanding beyond using the algorithmic rule.

You can also read how to subtract fractions with unlike denominators by reading the Quick Start Guide, "Adding and subtracting fractions" and how to check them in "The operations box: compare, add or subtract".

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