Factoring trinomials by first rewriting them

So back in December, I gave this problem to my students:

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To my surprise, it got a lot of traction with the kiddos. We spent the entire period talking about it. The idea was for them to see how rewriting a trinomial with four terms helps us to factor it. I’ve used this approach, often called the “CAB method” and used with a large “X” to organize the product and sum of the A and C terms, to factor trinomials for the last several years and I really like it for two reasons:

  • It doesn’t matter if a is greater than 1.
  • It naturally integrates factoring by grouping. Traditionally, grouping is learned after factoring trinomials. But with this approach, I teach grouping before we even see trinomials. Yeah:

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So, yeah, this is all great, but as I was explaining this approach to a colleague, she asked me why it works. It was in that moment that I realized that I had no idea.

Well, it turns out that later that day she went ahead and wrote up a proof of the method.

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I read a quote somewhere or heard someone say that the real usefulness of algebra is the ability it affords us to rewrite things in order to help reveal their underlying structure. This method surely epitomizes that idea.



3 thoughts on “Factoring trinomials by first rewriting them”

  1. This is so cool. I’m glad I ran into this post! The next lesson I’m planning for Algebra 1 (compiling materials, not to be taught till March) is factoring trinomials when a is not 1. Would be great to align with what you’re doing – what you called “CAB” I’ve called “AC” (..because its so *cool* — get it??), but kids would benefit from seeing it called the same thing.
    Also this proof is super cool. Hope we get a chance to catch up about it !

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