There is a procedure that people use and teach students to use for finding the inverse of a function. It goes like this (this image comes from page 10 of this document from Edexcel, but this pic is from Jo Morgan’s blog where I first saw it):

My problem with this is that it doesn’t make any sense, in two ways.

Firstly, why should this produce the inverse function at all? Why do you switch x and y? Why do you make y the subject? Why can you just declare “f-1=” at the end? I have seen any number of students in my drop-in centre who can do this, but have no idea what is going on or indeed what an inverse even is. Others know there is a procedure but struggle to remember what step comes next.

Secondly, the written working that goes with this procedure contains things that make no sense mathematically.


In this example, you write down y=3x+2 and then x=3y+2. I’d be wondering how it follows that if y is 3x+2 then you know also that x=3y+2. Indeed if both of these are true, then wouldn’t that mean you could sub one into the other to get that y = 3(3y+2) + 2? Which would mean that y = 9y + 8, and so y=-1. But wait, where did the function go? At the end, the “f-1(x)=” appears by magic. It seems to be replacing the y, but wasn’t y = 3x+2 at the start? This isn’t good maths writing!

I use a different procedure. It has one or two extra lines, but it makes more sense mathematically (to me anyway). Moreover, it just uses one fundamental maths problem-solving strategy, rather than being a procedure per se.

Here’s an example to show you how it goes:


There are two important things I want to say about this:

Firstly, my working includes the mathematical reasoning: the phrases with “let”, “then”, “but” and “so”. To me, this helps to make it more like proper maths writing. Also, I think it has a better chance of interpreted as reasoning rather than symbol-shuffling.

Secondly, the key strategy here is to give the inverse function a name “y” so that you can more easily reason about it in the hope to find out more explicitly what it is. This is a common maths problem-solving strategy and I think it is worth reinforcing much more than the inverse function procedure because it can be used elsewhere too! In order to do the reasoning, I have used the fundamental property of what the inverse function actually does, rather than just “switch x and y”.

If I had to write it as a procedure, this is what I would do:

  1. Give the function f-1(x) a name, like “y”.
  2. Do reasoning to figure out y in terms of x.
  3. You already know y=f-1(x), so you have the expression you need.
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This is another post about a teaching book I’ve read recently. This one is about the Which One Doesn’t Belong Teacher Guide by Christopher Danielson.

Picture from the publisher’s website

It goes with a beautiful little picture book called “Which One Doesn’t Belong?”, which is a shapes book different from any you’ve ever seen before. In this book, each page has four pictures, and asks the readers to say which one doesn’t belong, and why. The fabulous thing about the book is that there is at least one reason why each of the four pictures doesn’t belong, and talking through these with children (or indeed anyone) is a rich conversation about the properties that shapes have and don’t have.

The Teacher Guide is all about these rich conversations: why it’s important to have them, what you and your students/children can learn through them, and how to facilitate them. Chris has a friendly and welcoming style which draws you easily into a new appreciation of the sophisticated thoughts of children as they make sense of geometry and the world.

There are a few key things Chris talks about that really impacted my thoughts about teaching and learning maths. I’ve organised them by quotes from the book:

Commonly in maths class, student responses are compared to a standard answer key – the measure of what’s right is what’s in the back of the book, or what the teacher has in mind. In a conversation about a well-designed Which one doesn’t belong? task, the measure of what’s right is what’s true. — page 3

I read this quote first when someone else tweeted it out of the book and it struck me as awesome then. In my job at the Maths Learning Centre, students are always asking me if things are right, as if the measure of rightness is if I say it is. But in most places in maths, correctness is measured by truth. Your vectors will either be an orthonormal basis for the subspace or not. A number is either prime or it’s not. You can tell if you’re right by thinking about whether it’s true. I very much want to see opportunities to talk about the truth of things with students, to put the measure of rightness outside an authority figure.

“The van Heiles haven’t argued that it is difficult to go from level 1 thinking directly to secondary school geometry; they have argued that it is impossible. If students don’t have experience and instruction building informal geometry arguments, they will not learn to write proofs.” — page 8.

Chris is referring to the van Hiele model of “how childrens’ geometric thinking develops over time”. In this model, there is a build-up from noticing that shapes look like things they’ve seen (level 0), to noticing properties that shapes have and don’t have (level 1), to relating properties between properties of shapes (level 2), to logically supporting claims about these relationships (level 3).

The thing in the above quote that really struck me is the idea that it’s impossible to learn to write proofs without experiencing informal arguments first. I see so many students at university every day who struggle with proofs, and it makes me wonder that they maybe need more experience with informal arguments. Indeed, it makes me wonder if they need more experience simply noticing properties, since that’s an even earlier level. This is essentially applying the van Hiele models to other types of maths, but certain aspects of the progression still feel right to me, especially for things vaguely geometrical like vectors or matrices or graphs of functions.

I wonder if a student struggling with proofs might benefit from talking through a progression like this, and then helping them have experiences at the earlier levels before helping them with proofs.

“Of course being able to state new facts is an aspect of learning, but much more important to me is being able to ask new questions.” — page 21

I had never thought of this idea explicitly before, but immediately I saw that new questions were important to me as well. I was reminded of the time someone asked me if my students were understanding my statstics lectures. I said that I wasn’t completely sure, but certainly the students were asking very deep and complex questions. Instinctively I knew that a new type of question indicated learning.

Also, in the Drop-In Centre, there’s a certain joy when a student asks new questions you’ve never thought of before. They are wondering about the connections between things, which means they are learning, because learning is all about connections.

I am excited to listen out for new questions as a sign of learning, and to tell the students that it’s a sign of learning to have new questons!

“… I hope you will begin to see geometry through children’s eyes as well as through the eyes of a mathematician. Mostly, I hope you will come to understand that these two views of geometry are not nearly so distant as the school curriculum might lead us to believe.” — page 37

Now, I already believe that children’s investigations and ideas are actually very close to the way mathematicians work. You can’t be married to a very excellent early childhood educator without coming to some appreciation of this! It’s so nice to have someone publish a book telling teachers and parents the same.

Even more, this whole section is all about noticing and naming things and their properties. It’s about whether properties need names at all, or whether the objects that share those properties need names. It’s about what properties are important to make a thing a special thing and what aren’t, and in what context. It’s about the relationships between things. All of these are the work of professional mathematicians both pure and applied. And they are the work of children sorting out how the world works.

The geometry of children and the geometry of mathematicians are definitely not so far removed.

“I have come to understand that talking about this difference is more important than defining it away.” — page 54

Along with the rest of this chapter, this quote got me thinking about a whole new way to approach definitions in mathematics. As a pure mathematician, definitions are very important to me, and I always used to start with the definition. But I know those very definitions took years and even centuries to come to their current forms, and I also know that humans don’t learn through definition but through comparison of things that do and do not fit an idea. I think this is precisely what Chris is getting at here.

By skipping straight to the definition, we’re robbing people of a key part of mathematical thought, and we’re skipping them through the van Hiele levels before they’re ready. You don’t need a definition until you have a need to distinguish a thing from the other things around it. You don’t need a definition until you’ve noticed the properties you can use to define something.

The classic example in my own teaching is subspaces in linear algebra. The properties used to define a subspace aren’t even discussed until the definition is given. Little wonder, then, that the definition is meaningless to students!

It’s not just definitions either. I help a lot of students learn statistics, and one of the things that is never explicitly taught in your traditional statistics course is how to choose what is the most appropriate statistical procedure for the situation. I have been teaching this by focussing on some specific aspects of these procedures that statisticians use to distinguish things. Reading this chapter and this quote in particular helped me realise what I was doing was exactly “talking about this difference”. To distinguish between things you need to notice the properties that make them different, and to notice them, you need to compare things. I now have a much clearer idea of what I’m doing when teaching in the way I do.

I want to spend more time putting students in situations where they notice the differences between things and have to talk about them, so that they can distinguish between things they need to, and so that the properties I use to define things make more sense.

Thanks Chris for a most thought-provoking book.

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Writing about the teaching books I’ve read is fast becoming a series, because this is the third post in a row about a teaching book I’ve read.  The book I finished earlier this week is “5 Practices for Orchestrating Productive Mathematical Discussions” by Margaret S. Smith and Mary Kay Stein and coming out of the National Council of Teachers of Mathematics (NCTM) in the USA.

I’ll get straight to the point: everyone in any sort of classroom where maths happens should read this book. It gives a simple and practical framework for using student work and class discussion to promote maths learning. The authors have a direct, clear style that make the nuances of the practices seem almost obvious, using careful studies of classroom scenarios to illustrate. Let me say again: read this book!

In a nutshell, the idea of the book is that you can help students learn mathematical content by giving them tasks rich enough to be worth talking about and connected to the mathematical goals you have in mind, and then orchestrating class discussion of the methods students use and their connections. They give five practices, and a smattering of other strategies and ideas to guide this.

I think this book should be required reading and/or the basis of training for staff who are teaching tutorials at university. University tutors are often given no training in teaching, and even then don’t get tools to help them choose what to do in their classrooms. In some schools here at the Uni of Adelaide, they are instructed to get students working in groups. This is great, but the part where the mathematical ideas of the week are brought out is not strong. I am hoping to take these practices to these schools, and to the ones where it’s more just another lecture, in the hope I can help to improve the learning happening in the tutes. I’ll be mentioning how I think it applies to tutorials as I go.

Here’s a summary in my own words:

“Practice 0″: Worthwhile tasks and mathematical goals

You’re not going to be able to have a class discussion about a task which is routine procedure-following, because everyone will do it the same way. You need something that has some level of challenge and has decisions to make about how you do could do it — something actually worth discussing! Also, you need to have a goal in mind for what you want to achieve so that you have a chance of achieving something. This goal needs to be about the mathematical ideas involved. For example, about the connection between the different types of equations for lines, or about the distributive law, or about the relationship between squares and rectangles.

This isn’t technically one of the five practices, since it happens “outside” the context of the discussion. Plus, you may not always have total control over the tasks that students have to do or the mathematical goals. (More likely a school teacher is in control of this, but a classroom tutor at university this will be less often true.) Even so, if you do have control, it’s very important, which is why the authors call this “Practice 0″ a couple of times, because it’s needed before you even start.

As I already said, in classroom tutorials, someone else often chooses the tasks. But you can add your own question to the end to make it more open to discussion. Maybe something like “What would happen if…” are good to extend learning. Someone else may set the goal, but it’s more likely the people coordinating your course won’t tell you what the learning goal is. So you’ll have to choose for yourself. It’s so important to choose the goal so that the tutorial doesn’t end up feeling like a whole lot of activity and discussion, but with nothing of substance to take away.

Practice 1: Anticipate

When you have a goal and a task, the first thing to do is anticipate how the students will respond to the task. At the very least, you need to do the task yourself, but even better, imagine as many correct and incorrect, helpful and unhelpful approaches as you can.

One reason for this is so that you don’t have to make so many decisions on the fly during the class. You can figure out in advance some of the ways you will respond to these before you get there.

I see another advantage and it is about putting yourself in the mindset of your students. We university teachers are often so blind to how our students think, and tutors are often very focused on their own way of doing things. By explicitly trying to think of multiple approaches, it can help to break down this egocentric focus we fall into.

Practice 2: Monitor

Once you’re in class and the students are working on their task, the role of the teacher is to monitor the students’ work and thinking. The anticipating you did earlier helps you to respond appropriately to them, and sets you up into a mindset where you’re focused on their thoughts, so even unexpected methods are easier to process. It’s while monitoring their work that you will make the final decision of how you want to run the discussion, and who will be involved. It’s also while monitoring their work that you’ll ask the students questions to help them learn in-the-moment.

One thing I particularly like about this practice is how it gives us a focus while the students are working. Just the other day when talking to tutoring staff, they expressed a distaste for groupwork because it meant they, the teacher, weren’t “doing anything”. This practice says you’re not doing nothing — you’re monitoring.

The authors recommend asking students two types of questions during the working (and hence monitoring) phase:

  • Ask questions about student thinking
    Help students while they are working to express their thinking about the problem and the maths. Actually ask them to tell you how they are thinking. This gets them ready for the discussion to follow, and also helps them with the problem-solving too.
  • Ask questions about maths meaning and relationships
    Help students to express what the maths ideas mean and what they mean to them. In particular draw out relationships between concepts. This is what your goal is ultimately, and it front-loads this discussion so students are ready for it.

I see these two types of questions as really important for classroom tutors at university. Too often the questions we ask are about yes/no correct/incorrect answers, rather than about thinking and ideas. Encouraging tutors to focus on these types of questions makes thinking and meaning the focus of the learning activity.

Practice 3: Select

The last three practices are about making the discussion part of the class happen productively. They work together to help make sure that the discussion both uses student work, but also proceeds towards the mathematical goal. Also they prevent the random show-and-tell which often just ends up with students confused or with no particular idea of what they learned.

First, you want to select what student work you want to discuss as a whole class, and whose work it will be. The authors list a few considerations here, not least of which is choosing students who up to now haven’t participated much in class. It’s worth noting that in their examples, even though students worked in groups, specific single students are asked to talk about their work, which means people can’t hide from participating! It also means that people can’t monopolise the participation either! We all know that one person who seems to think the tutorial is just there for them to show how clever they are. By preselecting students to show their work, you’re making it less likely for this person to take over.

The thing I like most about the concept of selecting student work is that it has the potential to help students feel like their work is a valid and important contribution (which of course it is). By using student work and student generated ideas to forward the maths discussion, we can help them be more engaged in the learning and feel like we care about them. This is not a small thing to consider!

I am particularly interested in applying this idea to classes where students are expected to do preparation for the tutorial in advance and hand it in (like they do in several courses here at Uni of Adelaide). At the moment, what usually happens in these classes is that students do the homework, hand it in, and then the tutor presents their own preprepared solutions. But think what might happen if the students handed in the homework, and the tutor used the homework itself as a tool for class discussion. I think it might help the students feel like their homework was actually worth all the effort!

Practice 4: Sequence

After choosing which student work to present, you need to choose what order it will be presented in so that you can progress towards the mathematical goal. The authors give a number of things you might consider with your sequencing. For example, you might want to choose to start with a solution method that a lot of people have so that everyone can get buy-in to the discussion (I did this when I did Quarter the Cross in my daughter’s classroom). You might want to start with a solution containing a misconception to get it out of the way. You might want to avoid a specific solution because it will just send everyone off on a tangent (though you might also want to talk to that student one-on-one separately). You might want to have two particular solutions in quick succession in order to be able to compare them.

The important bit is to think about what order would be most helpful to get to where you hope to go. Importantly, the way you hope to make connections between ideas will dictate how you might sequence the students’ work.

Practice 5: Connecting

Now that you’ve chosen what student work to focus on in the discussion and in what order, it’s now time to actually have the discussion. It’s important here to remember there is a mathematical goal we’re working towards, which will often be about understanding a concept, and understanding is a sensation that happens when ideas are connected to other ideas. It’s our job to help students make these connections.

The authors suggest five “moves” you can make during your discussion to make sure it stays focused on the connections you want to draw out.

  • Revoicing
    This is when you repeat what a student says to make sure you and everyone else heard it and understands it. Importantly it’s not about you making what they said more correct, simply making it heard. A good phrase to end with when you revoice a students’ words is “Is that right?” This lets them know that the point is to make their thought heard (not yours) and they get to decide if it’s been voiced right.
  • Asking students to restate someone else’s reasoning
    Instead of you revoicing a students’ words, you can ask another student to explain the reasoning. This includes even more students into the discussion in a more active way.
  • Asking students to apply their own reasoning to someone else’s reasoning
    This time you’re not just asking the other student to explain the first student’s reasoning, you’re asking them to explicitly explain the connection between two different types of reasoning (one of which is their own reasoning). For example, suppose you’re doing Quarter the Cross and John proved the house-shape was a quarter by cutting and overlaying, whereas Jane proved the L-shape was a quarter by folding, you could ask Jane to prove the house-shape works by folding.
  • Prompting students for further participation
    There are times where a student will close off with a quick answer, and it might be more productive if they stayed in the discussion a little longer. The questions listed above of asking them to explain their thinking or focus on the meaning and relationships are useful now as well. In the Maths Learning Centre, I find “Tell me more about that” to be a good all-purpose request to participate further.
  • Waiting
    This may seem paradoxical, but leaving some silence can help to promote discussion. The authors say that whenever anyone asks someone else to say something, it’s appropriate to give them plenty of time to respond. Giving them this time helps to actually make the point that their answer is important to you. You giving yourself time to form your response to their question helps to make the point that their question is important to you. Waiting a bit after an explanation to let it sink in before asking people for any questions helps to make the point that it does in fact take time to process information. These last couple were new thoughts for me (though obvious in hindsight).

It’s this last practice that we often don’t do in tutorial discussions. I was talking to some tutors from the Faculty of Arts recently, whose tutorials are traditionally only discussion. They talked about how often the discussion just goes for a while and then stops at the end of the class, without coming to any conclusion the students can take away about the concepts or the process of learning them. They recognised a need to explicitly make connections during the discussion. Over in maths tutorials, I think we assume the connections are obvious, but I can attest that they are not, if all the students complaining that the tute doesn’t teach them anything are anything to go by.


It may seem that I’ve given you the content of the whole book, and indeed my aim was to present the ideas clearly, mostly for my own future reference! But I would still encourage teachers and tutors to actually read the book. The vignettes of actual classroom use are vitally important to come to an understanding of what the practices look like and where they are useful, plus there’s whole chapters about how to seek support for teaching and how to include it in formal lesson planning that I haven’t even mentioned (until just now).

I am excited to take the ideas here and use them to help support classroom tutors here at University. I think this book could really be a tool that people might actually get behind. Here’s hoping.

To wrap up, here’s the headings in dot point form for future reference:

  • Practice 0: Worthwhile tasks and mathematical goals
  • Practice 1: Anticipate
  • Practice 2: Monitor
    • Ask questions about student thinking
    • Ask questions about meaning and relationships
  • Practice 3: Select
  • Practice 4: Sequence
  • Practice 5: Connect
    • Revoicing
    • Asking students to restate someone else’s reasoning
    • Asking students to apply their own reasoning to someone else’s reasoning
    • Prompting students for further participation
    • Waiting
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Over the last week or so, I have been reading the book “Math on the Move” by Malke Rosenfeld (subtitled  “Engaging Students in Whole Body Learning”). Ever since connecting with Malke on Twitter back in June or July, I’ve wanted to read her book, and I finally just bought it and read it. Now that […]

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Over the weekend, I read “The Classroom Chef” by John Stevens and Matt Vaudrey. This is a post about my reaction to the book.
The premise of the book is to use cooking in a restaurant as a metaphor for constructing teaching in a classroom. It’s a good metaphor, and executed well. Warm up routines are […]

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What trig subsitution is
Trig substitution is a fancy kind of substitution used to help find the integral of a particular family of fancy functions. These fancy functions involve things like a2 + x2 or a2 – x2 or x2 – a2 , usually under root signs or inside half-powers, and the purpose of trig substitution […]

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This post is about a puzzle I’ve been tweeting about for the last couple of days. I got it originally from a book I was given back in the 1980’s called “Ivan Moscovich’s Super Games”. In the book, Ivan calls this puzzle “Bits”, but I don’t think that’s nearly descriptive or cute enough, so I […]

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It seems like ages ago — but it was only yesterday — that I wrote about differentiating functions with the variable in both the base and the power. Back there, I had learned that the derivative of a function like f(x)g(x) is the sum of the derivative when you pretend f(x) is constant and the […]

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Here in Australia, we are at the tail end of a reality cooking competition called “Zumbo’s Just Desserts“. In the show, a group of hopefuls compete in challenges where they produce desserts, hosted by patissier Adriano Zumbo. There are two types of challenges. In the “Sweet Sensations” challenge, they have to create a dessert from […]

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In Maths 1A here at the University of Adelaide, they learn the following theorem (this is taken from the lecture notes written by the School of Maths here):

It says that, given a function of x defined as the integral of an original function from a constant to x, when you differentiate it you get the […]

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