How do you teach this topic so that students don’t make this mistake? (Or do you just rail on their earlier teachers?)

Thanks to Kristen Fouss for the submission.

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- Post author By mpershan
- Post date January 30, 2013
- 2 Comments on Canceling and Trig Identities

How do you teach this topic so that students don’t make this mistake? (Or do you just rail on their earlier teachers?)

Thanks to Kristen Fouss for the submission.

## 2 replies on “Canceling and Trig Identities”

I don’t allow the word “cancel” to be used in these situations. They have to say a whole big sentence like “Dividing both sides of the equation by …” or “dividing top and bottom of the fraction by …” or “because these are inverse functions…”

I also tell lots of jokes like 16/64 = 1/4 (by canceling the 6) and sin x / n = 6 (by canceling the n) and so on. It’s always illuminating to see whose number sense treats the “canceling the 6” operation as potentially plausible.

Oh boy do I see this a lot. I’ve adopted Bowman’s DPT (well, I’m not sure it’s his, but he made these cool posters: http://goo.gl/OOjAtM) for convincing students not to do that.