Crutches and Exponents

Math teachers frequently discuss how students forget what the exponent rules actually mean when they make mistakes applying them. The layer of abstraction that these rules lay over the numbers and operations is at fault, of course. The reason we teach the rules is that they show structure that goes beyond the operations. They simplify our work in calculating expressions.

I was really glad that a student used this approach today when she forgot the rules:
Screen Shot 2015-10-29 at 9.38.07 AM

I would much rather a student move back to a method they know rather than blindly apply the rules they don't. This method, or crutch, is less efficient, but holds more meaning for the student. We dissuade students from crutches like counting on their fingers because they should be able to do the arithmetic in other ways. Building meaning is important, however, and the better approach would be to show how learning the mathematical ideas and structures can simplify the process. In speaking with this student afterwards, it was clear that going back to this method that we used to motivate the rule helped her understand what it meant.

I continued with this approach in reviewing zero and negative exponents today. Of the students that said they knew the rule already, only a couple of them actually applied it correctly before we did this activity. I primed the class with this:
Screen Shot 2015-10-29 at 9.41.32 AM

Students worked in groups to apply the rules and rewrite them, and I nudged them gently with using what they saw as motivation for rules about zero and negative exponents. From this, I introduced a new crutch as a way to show what negative exponents mean:

Screen Shot 2015-10-29 at 9.43.48 AM

Just as the student wrote out the factors and then divided them out in the problem above, I don't mind if a student does this as a reminder of what the rule means. I find this much more productive than a simple rule that states that fractions to a negative power simply 'flip'. Hopefully I'll see the benefits of this approach moving forward.

3 thoughts on “Crutches and Exponents

  1. Thanks for that reminder - I feel the same way about crutches.
    Interestingly, I've heard of a few studies linking finger counting and number sense, like this one (http://www.ncbi.nlm.nih.gov/pubmed/18387567). It seems that for all we do to stop kids from using the "crutches" like writing things out and finger counting, we might actually be crippling them.

    1. Hi Lisa,

      Thanks for sharing the study - certainly news to me. I wonder if there is a connection between pushing students to abstract reasoning too early and the difficulty these students have later on in understanding what they are doing in mathematics. I see the need for the abstraction, but not necessarily when it occurs in the traditional school sequence as it exists now.

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