In Geometry this year, I wanted to write a few more problems to have kids practice with angles of regular polygons… so as I was coming up with a few problems, I realized they had a nice theme to them.

*“Which polygons fit together snugly? Which don’t?”*

[02 Snug Angles download][Note: There is a typo on #6… It refers to problem 2d, but it should refer to problem 2c]

I made this the day before the class I was going to teach it. But I wanted to have a hands-on “playful” component to this. I asked teachers in my school if they had regular polygon tiles with the same side length… I got a set which included triangles, squares, and hexagons. No pentagons, no heptagons, no nonagons, no decagons, nada.

Of course these tiles were probably produced for lower school kids precisely because they fit together “snugly” at a vertex. But no “play” could really happen if sometimes things *didn’t* fit nicely together. So — for future reference — I asked on twitter to my math peeps if anyone knew where I could buy regular polygon tiles of all sorts. No links were forthcoming. Sigh.

In class, I expected #1 to be challenging. I wanted students to come up with a *reason* they had found all the possible regular polygons (of one kind) that fit snugly together. It was nice to see students reason through it, and when we came together as a class, we had a few different cogent explanations. Some involved calculating all possible factors of 360. Some involved recognizing that the more sides you have in a polygon, the fewer of them can fit together “snugly” at a vertex (and the minimum number of polygons that can fit at a vertex is 3).

Although I was expecting #2c to be challenging, I didn’t realize how challenging it would be. I thought I had built a scaffold with the previous problem so it wouldn’t be too hard. What turned out to be the problem? The fact that a regular 7-gon had a non-integer interior angle value. Kids didn’t know that could happen, and that really threw them. Also: setting up the equation was challenging, because kids were confusing “the sum of the interior angles in a regular n-gon” with “the measure of *one* interior angle in a regular n-gon” (a calculation they had never been formally taught, and were supposed to figure out themselves during this exercise).

I’d say only about half the groups could deal with 2c without any help.

However, all groups ended up being successful. And I just graded their assessments on polygonal angles, and almost *every single student* got the problem that was similiar to 2c!

The very last question asks students to discover as many possible combinations of regular polygons that could fit together snugly at a vertex. I assigned this as a nightly work problem — and the next day, students came in with lots of great combinations. Unfortunately, I didn’t do anything with this. I should have — but I felt pressed for time.

We could have talked about why 6 polygons were the maximum number that could fit together, or 3 polygons were the minimum number that could fit together. That could reduce our searching! Then I could have asked how people approached the task. Guess and check? Geogebra? Is there a systematic way they could have approached this problem — if they had infinite time and patience — that they could guarantee they had found all possible combinations? Do all combinations need at least one 3, one 4, one 5, or one 6?

Or we could have spent some more time looking at all possible combinations. Some kids noticed — after looking at the comprehensive list I threw on the board after they finished sharing their values with me — that many of the values had common factors: so 3, 7, 42 is one crazy combination that works. And both 3 and 7 are factors of 42. What else could we find?

What I’m trying to say is: the last question was kind of a dumb question to put on the sheet without having a good way to debrief it, and a meaningful conclusion we could have gotten from it. Sigh.

Okay, on to the exciting part. I said I asked on twitter if anyone had a site to buy these tiles. No responses. *BUT* Christopher Danielson then asked what I was looking for. Kate Nowak jumped on the bandwagon and brainstormed what a teacher might want, ideally. Yesterday, I came home from school and had a box waiting for me. In it:

They are *beautiful*. And gosh do they smell awesome. Real wood, that smells awesome. I was in heaven when I saw them. So beautiful.

And even more satisfying: you’ll notice that the 3, 7, and 42 fit snugly together!

Now the million dollar question: assuming I had however many of each tile I wanted, what would I do with them? How would I restructure the unit to use them in a way that is compelling? I wanted the tiles initially because I thought some “play” with the tiles would be fun, before delving into the algebra to see the justification of why some work and some don’t work. But I want something more! Something that will have them figure out the 3, 7, 42 connection and gasp! And the 4, 5, 20. And the 3, 8, 24. And the 3, 10, 15. And the 4, 5, 20. And GASP with surprise and horror and delight!

I don’t quite know… But maybe envelopes with index cards in them. And some of the index cards have some configurations they have to “check” to see if they work or not. And some of the index cards have two of the tiles, and students have to see if there is a third tile that works. And for each configuration that works, students get to come to the front of the room, grab those tiles, and check to see if their algebra worked by checking to see if the tiles truly do fit snugly. If they do: they record their discovery on the board for all to see. And by the end of the class, students will have had practice, and in the last 5 minutes, we could all gather at the front, and view some of the weird snug angle configurations together. And see how configurations that are “close but no cigar” don’t work (like 3, 10, 16… which is close to 3, 10, 15). When doing this, we could also talk about why 4, 10, 15 is “worse” than 3, 10, 16 in fitting snugly.

That’s all my musings for today! I’m going to be chaperoning a trip to Spain in a few days, and that will last two weeks, so goodbye for a while!

SUPER-SUPER-SUPER-LIKE! YAY, MTBOS!!!!!!!

– Elizabeth (@cheesemonkeysf)

Those are absolutely beautiful!

Enough to make any geometry teacher mucho jealous!

And all spawned from a wonderful question: “Which polygons fit together snugly? Which don’t?”

Not that this is compelling, but I wonder if they could design a better soccer ball with polygons. The traditional soccer ball is composed of pentagons and hexagons. What’s the relationship there?

I like that this will lead into polyhedrons easily as well…

Also, I got a total of 17 different sets that fit snuggly. Did I miss any?

(A little algebra in a spreadsheet).

You can check if you got ’em all here! https://en.wikipedia.org/wiki/Euclidean_tilings_by_convex_regular_polygons

If you used google spreadsheets, could you share your spreadsheet? I’d love to see how you approached the problem!!!

I would love to see that spreadsheet too. #DiscreteGeometry

I wonder if it would help my pre-calc students, as we begin trig, and they’re thinking about angles, to do something like this… Hmm…

Here is the link to my spreadsheet:

https://docs.google.com/spreadsheets/d/1pX7tGmuQnp6pLSwNwhzVkWmG19WnQE1n2lBf-qwsRRI/edit?usp=sharing

It is not very automated. Basically just allows me to start filling a vertex and have the numbers in front of me to make it easy to decide what else to add.

This post makes me very happy, Sam! They were a ton of fun to make. We’ll work out all the lesson planning details while you’re away. Safe travels.

Awesome!

If you don’t have those tiles, you can get students started thinking about this with my Geometry Labs template (available from Nasco). It includes regular 3-, 4-, 5-, 6-, 7-, 8-, 10-, and 12-gons. See Lab 7.4 in http://www.MathEdPage.org/geometry-labs/ (Related labs, leading up to this: 1.1, 3.5, 7.2, 7.3. All are free downloads.)

Those tiles are amazing! Just examined that problem in a class a few weeks ago and scoured the internet for such things, even paper cut-outs with congruent sides. Are they laser-cut?

Also, one of the best problems of PCMI 2014: “For each triple [of number of sides] you found in [previous problem], there’s something interesting about the sum of the numbers, the product, or the sum of the reciprocals. One of those.”