Tag Archives: geometry

Geometry and language

Interesting conversation on Twitter today with Bryan Meyer, Denise Gaskins and Justin Lanier. It began with these tweets on my part, the result of grading some student work.

Things quickly got too nuanced for Twitter.

An example of something my students struggle with is answering a question such as, Is a square a rectangle?

This type of question asks about class inclusion. Is an element of a subset also an element of the larger set?

Many useful and interesting questions in geometry have to do with whether one class is a subset of another class. Do all isosceles triangles have a pair of congruent angles? Are all quadrilaterals formed by connecting midpoints of other quadrilaterals parallelograms? Are all Stacys concave?

I am trying to sort out the extent to which my students’ struggles with questions of this sort are linguistic, and the extent to which they are about struggles with the idea of class inclusion.

Justin suggested this wording, which I will investigate:

Is a square an example of a rectangle?

Or, more generally:

Is an X an example of a Y?

My suspicion is that this will be helpful for some students when asked in this direction. But I also suspect that asking it in the other direction will be problematic.

Is a rectangle an example of a square?

See, part of what I wonder about is whether class inclusion—and the fact that it doesn’t have to be symmetric—is at the heart of a particular kind of struggle in geometry, and whether this is also related to the ways students think about and use language.

I hope these three (and others) will weigh in here where we have more space to work than we do on Twitter. The ideas are really useful. If you’d like to follow the prior discussion, you can follow this link.


Van Hiele levels in my geometry instruction

This mathagogy video has generated some behind-the-scenes discussion in recent months.

This week, I got a note from Chris Hill with some questions. I wrote him a long email in response. Here it is:

First a disclaimer.

I have never taught high school geometry.

I taught middle school, including some geometry there, have written some middle school geometry tasks, and I now work with future elementary teachers. That last is the context in which I developed my hexagon thing. The sequence I describe in the video is the end result of several years worth of trying to work out a way to get my students to geometric proof.

Of course there is a lot more involved in the process than can be conveyed by a short video. Have you seen my blog posts on the sequence?

And now your questions:

Do students develop the ability to move up the levels through the entire year, or on each unit?

I think of the van Hiele levels more as an organizing principle for instruction than as a principled diagnostic for students. But the levels do serve both purposes.

Here is what I mean…I do think that most of my students (high school graduates; some young, some older; all students at my community college) come at a pretty easy to observe Level 1. They know names and properties of shapes but cannot talk much about how these properties relate to each other. That, I think, is pretty standard for entering high school geometry students too.

I do not think that they become Level 3 geometers over the course of our work.

Instead, I use the van Hiele levels as a way to organize instruction. So we start where they are with some noticing and naming of properties. Then we move to a tremendous amount of level 2 work. I probably describe some of that in the blog posts, and I should probably write up more of that.

Over the course of several class sessions (we meet for 80 minutes twice a week), I push a little bit harder, and then harder still, on the arguments students are making. Eventually, through this modeling and through my explicit talk with them about it, they begin to be more critical of their own arguments, and eventually we reach a question that sort of requires proof; it seems true, but is non-obvious, and it has arisen from the questions we have been asking (at level 2) about how properties relate to each other. Creating and organizing the hierarchy of hexagons is often a useful part of this sequence.

And then we move on. If we had the whole semester (or whole year!) to spend on geometry, I would imagine that level 2 work would be an important introduction in each unit. This is Serra’s conjecturing phase in the Dan Meyer comment.

Is it reasonable to expect every high school geometry student to get to the top two levels within a unit (like circles)?

I don’t think the top level (level 4, rigor) is a reasonable goal for every student. Sure, you can do Taxicab Geometry and notice that “circles” there look like Euclidan squares, but I am not convinced that this has a lasting effect of rigor on students.

I do think having all high school students (for whom future elementary teachers are my proxy) use their ideas about relationships among properties of shapes to build a more formal argument that a proposition must be true is reasonable. Different students will be capable of different levels of sophistication in terms of the propositions they argue, and in terms of the arguments they make, but they can all do some level 3 work for sure.

If I try to talk about the levels explicitly, do you think high school students will be insulted, not care or would they feel validated when they take a long time to get to a higher level?

This is a good question. I debate this a lot in my own teaching. How much of the motivation and justification should I provide my students for what we are doing? I tend to talk about things in that spirit—we are doing X today because it will address needs that I know we have; here is how I see this activity getting us to our goals. And I try to provide specific evidence and the source of that evidence. A research basis is a good source of evidence, as is student work on quizzes and things kids say in whole group discussions.

I would not spend time telling students what van Hiele level they are at. And I might not use the actual language of the van Hiele levels with kids (but then again I might).

Rather, I might say something like this:

Our goal is to make formal mathematical arguments (proofs) in this course. Mathematical arguments take a different form, and have a different standard than a scientific argument or an argument you may have with your mom. But that is a goal, not our beginning place.

I know from research I have read, from my experience in previous years, and by observation of your work and the ways you talk about geometry that an important part of getting to that goal will be exploring properties of shapes and how those properties relate to each other. So we will do a lot of that work.

We will argue with each other. And over time, I will help you to make these arguments more mathematical. Eventually, you will be able to make such a great mathematical argument that we can call it a proof. Some of these proofs will be developed and written by the class collectively, and I will ask you to try do write some on your own. But it will be a while before we get there.

Then, when we do get an example of a solid mathematical argument (whether because the class developed it or because I provide it), I make sure to stop, notice it and to have a conversation with the class about how that argument is different from the other work we have been doing.

Do you know of any standard assessments that I can use to track my students’ progress with these levels?

No. I ask my students to make short arguments on quizzes, and try to pose questions that admit both informal and more formal arguments. But I don’t know of good vH level instrumentation. It may exist. But I don’t know about it.

Another cool thing…

In our discussion the other day of whether the difference between van Hiele levels 0 and 1 is the use of official math vocabulary terms, or whether there is something more there, a student asked this (I am paraphrasing):

Is this like the CGI research, where children know things about addition and subtraction before they are taught in school?

Here’s what I love about this question:

  1. It demonstrates the power of having these students for a second semester;
  2. It demonstrates that students will—over time—make these ideas their own; if we keep at it, and build cases for the importance of the ideas in our courses, students will play with these ideas, and they will look for opportunities to apply and connect them; and
  3. It made me stop and think. I’m still not 100% sure whether it’s like that or not. I have something new to think about.

Defining concavity

Today was day 2 with my geometry students/future elementary teachers.

Homework due today consisted of (1) a reading about the van Hiele model, and (2) participation in a discussion on Canvas about geometry terms; they were to choose one term from an extensive list we generated whose meaning they are confident of, and whose meaning they do not know well.

In that discussion, several students chose concave as the term whose meaning they did not know. Another student replied (for they are required to reply substantively to at least two other students’ posts):

Concave is a shape that curves or angles inward and convex is like most regular shapes, and has all its angles pointing outwards.

Now class time is about integrating ideas (does it count as flipping the classroom if they read first, then discuss in class?) So they were given the task of deciding which van Hiele level best describes this claim about the meaning of concavity.

The brave soul who offered her take said, (and I am paraphrasing):

A major difference between level 0 and level 1 in the van Hiele model is whether we are naming the properties; whether we are using geometry vocabulary.

Lovely. This gave us something to work with.

I proposed a game in which we delete all of the official mathematics vocabulary words, replace them with more informal language and ask whether we have changed the van Hiele level. Now we had:

This is a shape that curves or points inward and that is like most normal shapes, and has all its corners pointing outwards.

That first brave volunteer felt that the nature of this claim is now different; that this edited version is a level 0 statement, while the original is level 1. Others were not so ready to commit in either direction. I argued that this second claim has a similar structure to the first, and so really ought to be at the same level.

To illustrate the nature of the claim, I asked them to explain what is meant by pointing inward, or angling inward. 

Pointing towards the center of the shape was a definition they could all agree upon. I drew a few examples of concave shapes whose concavities didn’t really seem to point towards the center of the shape, they drew a few examples and they began to formulate other ways of saying what concave means.

20130117-152228.jpgIn the end, we agreed that it’s awfully difficult to characterize pointing inward in a rigorous way, and that this may not be the best way to state what concave means.

And we agreed to revisit this term later on.

I suppose I left dangling the fact that an important part of the structure of the original claim is that it rests on terms whose meaning is imprecise to the speaker. It’s not about whether I use the word angle instead of corner; it’s about whether I have a precise and defensible meaning for the word I am using.

Do I mean for my word to point to a particular class of objects, while excluding others, and can I tell the difference between these two classes in a principled way? That is the difference we need to unearth.


Hierarchy of hexagons follow up

Short note with a couple of important points about my hierarchy of hexagons post.

  1. An essential piece (perhaps the essential piece) of this whole teaching sequence was that it was based on what my students saw in those hexagons. I designed a diverse set that I thought had interesting properties, but this wasn’t about them guessing my properties. Instead…
  2. This was about my helping them to better articulate exactly what they saw in these hexagons. I helped them to develop good language for describing properties (this is not easy to teach, nor to learn!). And I helped them to understand differences between properties and definitions (again, not easy for either party).
  3. Josh wrote in the comments, “I would have worried about leaving so many types of hexagons uncategorized or about not recognizing some of the important types.” Which reminds me to state that this activity is all about process, and not at all about content.
  4. Perhaps that last claim is overstated. I had a few items of content to sneak in there. Vocabulary such as concave. The difference between equilateral and regular. That sort of thing. But there was no one thing about hexagons that I needed to get on the table. Instead, hexagons were our territory for exploring mathematical practices.
  5. This was my second time through the activity. Last semester, my students noticed somewhat different things in the hexagons, and we ended up with candlesticks, chevrons, shields, starfish and rectilinear hexagons. Again, the exact properties and resulting categories aren’t what matters here.
  6. I did a much better job of integrating the hexagon sorting into the proof work. The Venn diagrams and the hierarchy were important to that.
  7. If you read carefully, you’ll see the van Hiele model play out pretty tightly as the underlying model for the sequence of activities. We did not proceed to Level 4.