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Ellen's Choice: Teach the Concept, Not the Algorithm

I hear a lot about math teachers from my students, and while every teacher is unique, some comments are repeated over and over. By far the most common one I hear is that their teacher didn't really explain something, or was incapable of elaborating when questioned and simply repeated the same lecture again. As a tutor, my first priority is to make sure the student understands the material, and if they're still confused, to find another way to explain it so that it makes sense. In order to do that, I need to have a thorough understanding of the concepts myself, so that I am not simply reading from a textbook but actually explaining a concept. In my years of tutoring math, I've developed a point of view and approach to math that I refer to as “teaching the concept, not the algorithm.”

An algorithm is a step-by-step procedure for calculation. The term is used in math and computer science, but the concept of an algorithm is universal. I could tell you that I have an algorithm which consists of 1) close the windows; 2) put on a sweater; 3) check the thermostat; 4) turn it up 3 degrees if it displays lower than 68. This is pretty obviously an algorithm to solve the problem of “I am cold right now.” We have algorithms for everything in our life, and most of the time we don't even think about it that way. We see a problem, we work out a set of steps to solve it, and we complete those steps and observe the result.

In math class, however, students frequently encounter teachers who simply teach the algorithm; handing them a formula for solving a problem without ever really teaching them the core concepts involved or why the formula is what it is. This results in a lot of rote memorization with no understanding of why the numbers are where they are in that formula. My golden question for math teaching is always “Why?” Why does this work? Why can I do that? What am I trying to accomplish here, in the grand scheme of things? If I can explain the concept to the student so that they understand what they are doing on a macro scale and why their actions work and make sense, then it doesn't matter if they forget the formula itself, they should be able to figure it out organically by going through the conceptual process again.

I'll give you an example from my favorite math teacher, Mr. Lazur. (I wrote a whole blog post about Mr. Lazur's teaching style, which heavily influences the way I tutor.) I had Mr. Lazur for Geometry, a subject notorious for the amount of formulae it throws at its students. Every single type of shape has three or four formulas associated with it, and keeping them all straight can be a nightmare for students. Mr. Lazur got around this by showing us WHY the formulas look the way they do, ensuring that his students could always reverse-engineer the formulas from the concepts if they couldn't remember them directly.

In this example, we're learning about the volume of a cylinder. We've just spent the previous few days discussing volume of cubes and rectangular prisms, so Mr. Lazur starts us off by reminding us of exactly what volume means. It's the amount of stuff required to fill up the shape; the amount of water that would be displaced if the shape were dropped into a bucket. Then he pulls something out that nobody was expecting: one of those CD spindles that you buy with blank, recordable CDs in them. He points out that a stack of CDs is a cylinder, taking them off the spindle and setting the stack on his desk. He asks us to imagine that each CD is actually a truly 2-dimensional object, ignoring the tiny thickness of the plastic. He tells us that the process works just the same with truly 2-dimensional objects as it will with these CDs. How could we figure out the amount of stuff required to fill up this shape, he asks. Assuming it was truly 2-dimensional, we wouldn't be talking about volume anymore; it'd be area, right? He asks for the area of a circle, and we give it to him. We know this; it's easy stuff we've known for months now.

A = πr2

So that's how much space this single, 2-dimensional CD takes up, right? He picks up another CD and places it against the first one, flat sides together. How much space would 2 of them take up? He separates them again, holding them side by side. It'd just be twice the amount of space the first one took up, right? 2 circles' worth of area.

He writes on the board: 2πr2

So how much space would 5 of them take up? 5πr2

And how much space would a stack of them that was h CD's high take up? hπr2

Mr. Lazur then circles that last line and turns to us. “This is the formula for volume of a cylinder. It's just the area of the flat face, multiplied by the height of the stack of those faces. πr2h.”

When I started writing this blog post I wasn't thinking about the formula πr2h – I was thinking about that stack of cylinders. The formula followed organically from thinking about the concept. And that's the key – you can derive an algorithm easily from a concept, but if you never teach the concept all the algorithms in the world are just meaningless memorization.