Triangle sort

Pretty standard activity, but I liked this one a lot. First I wrote each of the sets of numbers below on sticky notes.

Non-triangle Right triangle Non-right triangle
8, 9, 17 5, 12, 13 9, 10, 15
8, 12, 20 6, 8, 10 9, 14, 15
3, 6, 11 3, 4, 5 5, 12, 15
6, 7, 16 9, 12, 15 8, 12, 13
8, 12, 21 9, 13, 2√22 4, 2√55, 16
1, 6, 7 5, 10, 5√5 4√2, 6, 9
6, 11, 19 2√2, 2√2, 4 5, 4√10, 14
4, 6, 12 √57, 13, √226 √5, √10, √14

A bad picture of some sorted sticky notesWhen students came in for the day, I chose a sticky note for them (woohoo, differentiation!) and told them to wait for further instructions. When class began, I told them to figure out if their set of three numbers represented the side lengths of a non-triangle, a right triangle, or a non-right triangle. Once they were confident in their answer, they were to put the sticky note in the appropriate location on the window.

Once all the sticky notes were placed, I looked to see if they were all in the right spot. If there were any mistakes, I had everyone grab a different sticky from what they had before, and check whether it was correctly classified. Then we repeated the process until all were correctly classified. Students were encouraged to work together to make sure they knew how to categorize three side lengths—and they actually did!

This was on an otherwise-individual work day, so after students had placed their sticky note on the window, they worked independently on other work. I definitely recommend doing it that way if you’re going to be mean like I was and insist on 100% accuracy without giving any hints as to which sticky notes are incorrectly categorized. Also I used my super-sticky Post-its, which are awesome and withstood an entire day of sticking and unsticking.


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