I’m posting about this activity that wasn’t really a good activity (some may even say it was bad) because maybe someone can use it and make it better.

I’m teaching a similarity unit that is very boring. Ratios/Proportions, Similar Polygons, Proving Triangles Similar, and Proportions in Triangles. No mention of transformations, no scale factor, no area/volume…and on top of that my students have never seen anything about congruent polygons. So I was looking back at my old geometry files (yay for finally teaching something I’ve taught before!) and I adjusted my notes packet and printed that for the kids. They love that their notes are all in a packet. But every day was just notes and practice, notes and practice, notes and practice. I was so bored. Kids were bored (truth: some were probably thinking this is normal). So I remembered my old activity where students discovered the triangle congruence theorems and thought I could possibly make that activity work for similarity.

It didn’t really work…Here were the problems:

- Building the triangles takes sooooooo long. I wish I had read my old post more carefully and believed that it would take so long. I cut it down to 4 triangles and had groups of 3-4 and it still took about 35 min just to build all the triangles. Why does it take so long?
- I didn’t have a lot of color paper to use so it was hard to see the cutouts on the board.
- I had two different groups so there should have been proportional sides. I only told that to my last period for some reason, so the other periods didn’t really get why triangles were similar
- It’s a lot easier to see that triangles are the same than see they are similar – you just kind of have to guess and a lot of them didn’t trust their intuition that we could rotate and dilate (probably because it wasn’t ever discussed)
- Once one triangle was put up for my SSA example, the other groups would just make the same one that they saw so some classes ended up thinking SSA made similar triangles

IF I was to do this again (which I don’t think I will), I would:

- Not have students put up their triangles until the very end
- Demo building the triangle (only thought of this in my last class)
- Use color paper
- (Maybe) ask kids to specifically check for similarity – congruent corresponding angles and proportional corresponding sides. This takes some away from the discovery but would make it less of a guessing game. It would also take more time.
- Plan for it to take a lot of time

Here’s the files just in case they’re useful to someone:

Similar Triangles Discovery.docx Similar Triangles Discovery.pdf

N’enseigne pas cette leçon sauf si vous le modifiez.