The students are trying to prove why the finger trick of determining the multiples of 9 works. We see one student explain her thinking. I ask if someone can repeat her reasoning. As the second student revoices, I silently represent her thinking on the board. Notice that I do not evaluate in either case whether the thinking is correct or complete. Instead I continue with a new question that connects this idea back to the original problem.
This video was taken in a mathematics for liberal arts class with 15 honors students. It met two times a week for 75 minutes each.
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