Gotta love the distributive property. Have the student substitute values for a and b in both their incorrect answer and the original problem. Let them see the two different results. Proceed from there.

Substitute values in like Chris said. Also, I’d remind kids that a could mean elephants and b could mean muffins. However, I’m more focused on the word “multiply” in that problem, and I don’t think it should be there. What do you think?

Yeah, Fawn, I was so focused on the mistake that I didn’t even notice the multiply in the directions. I think that your observation would make a good blog in its own right. We submit our own or others problems/assessments/whatever and receive critical feedback.

Jinjer Markley

It’s tempting because it’s so much easier than distributing!

I like to teach my students to doubt their memory of math rules, and check them with numbers *before* doing a variable problem. Can you just make 3a +4b into 7ab? that would be like 3* 1 + 4*2 = 7*2. does 3+8=14? Nope, so there’s a different rule. What is it?

Kit

One thing I like to do is use algebra tiles. Particularly if you have an x tile and a y tile that’s a different “unknown” length, I talk about how many tiles of x and how many tiles of y do we have? And then, I point out that adding them doesn’t change the type of tile we have – so it’s still x, and still y. If we’ve got an xy tile, that’s a DIFFERENT tile, so renaming it ab is like calling over the xy tile

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