In April, I asked my Accelerated Algebra 1 students to write an essay. The prompt was simple:

Compare linear and exponential functions. Explain the essential characteristics of each, and discuss what is the same and what is different about them.

I’d thought about having students write an essay in the past, and one of my colleagues had assessed her students using a similar prompt a few years back. Assessment wasn’t really my goal, though. With months of work with linear functions under their belts, my students could calculate slope, write equations for lines (in multiple forms!), determine where two lines intersect, and use linear functions creatively (Desmos art!) and practically (modeling-type problems). But when I asked the class one day about the key feature of a linear function, they didn’t have a great answer for me. Their answers were okay – “it’s a line!”, ““, “there’s a slope” – just not quite what I was looking for. The last response came closest, but my follow-up question of “What does it mean for a linear function to have a slope?” didn’t elicit the sort of responses I wanted.

A linear function has a constant rate of change.

Eventually, someone said that, and I probably shouted in joy. You might prefer to describe the essence of linear functions differently, but for my purposes, “constant rate of change” is the key phrase. And it’s one my students knew. They just didn’t think about it as much as they thought about all the procedural stuff. So I said to them:

More than anything else, I **need** you to finish this course knowing that a linear function has a constant rate of change.

Sometimes, I think, we get hung up on all of the skills and techniques and nitty-gritty details, so hung up that we forget about the essence of our content. It’s nice to be able to write an equation for a linear function, but in my eyes, it’s crucial to be able to know that a linear function can model a phenomenon with a constant rate of change. I had to make sure they owned this.

I decided an essay would help me achieve this goal for two reasons. First, an essay would force students to refine their thinking and work toward understanding and explaining key concepts. They couldn’t hide behind procedural fluency. Second, because a math essay seemed so strange, students would approach it differently than they approached other assignments. Writing involves skills and thought processes that we don’t often use in math class, and I felt like writing about math would help students gain a new perspective on the content.

I wish I could say that my students loved the idea of writing a math essay, but they didn’t. Students expressed a mixture of confusion and anxiety. To their credit, however, they complained very little. And for the most part, they completed their essays on time.

Here are some guidelines I offered:

- Strive to make your writing feel like an essay. Don’t just give me a list!
- One paragraph can be enough. You may write more, but you don’t need to.
- Be sure to use mathematical vocabulary.
- Use examples judiciously. I want to read about linear and exponential functions in general, not just about a specific linear or exponential function.
- Feel free to supplement your writing with equations, tables, graphs, and images.

Just for good measure, I also provided an incredibly vague rubric.

A: Beautiful essay that demonstrates exceptionally deep understanding of linear and exponential functions and moves MC to tears.

B: Solid essay that demonstrates strong understanding of linear and exponential functions and makes MC feel pretty good about life.

C: Okay essay that demonstrates decent understanding of linear and exponential functions but that has some weaknesses that leave MC feeling unsatisfied.

D: Inadequate essay that demonstrates little understanding of linear and exponential functions and makes MC wonder how much you’ve really learned.

F: Complete lack of an essay leaves MC curled up in a ball in the corner of his classroom. Please don’t let this happen!

Then, I waited. A few students asked for advice, and I read through several essays so that students would know if their essays met my expectations. Interestingly, several students asked the English Language Arts teacher on my team to look over their essays. Although I was a little apprehensive about how the essays would turn out, I mostly felt good about the thought and the effort that I saw.

The deadline passed, and it was time to read the essays. Here are some excerpts – some great, some not great – that I found interesting. All spelling and grammar has been preserved from the originals.

When you deal with a linear function you add the same number each time, but an exponential function multiplies by the same number each time instead of adding.

In an exponential function the line is curved meaning that the rate is not constantly the same.

But from the beginning you can tell that they will be different from “line” and “exponent” in their names.

Another major difference is that exponential functions never ever, reach zero, this is called an asymptote.

An exponential function is a function that increases at a constant rate raised to a power. It is important to know that, in an exponential function the independent variable is the exponent and the base was consistent.

Instead of having a constant rate of change the exponential function changes by a common ratio.

In a linear function the line touches 0 but in a exponential function the line never touches 0, it’s an asymptote!

Because the rate of change is not constant exponential function are able to increase/decrease faster than linear functions do

Exponential will have a slight curve in it and will eventually get super steep.

linear functions unlike exponential functions have x intercepts, due to the fact that exponential functions are unable to ever reach zero because the amount will keep getting cut in half.

Another similarity is that both of the domains are all real numbers for both functions.

The main difference between these two types of lines is that exponential lines slope is increasing by a certain percentage each time whereas linear lines have constant slopes.

linear functions are arithmetic (adding the same number each time) and exponential functions are geometric (multiplying the same number each time).

Another similarity is that both of the functions intercept the y-axis and the x-axis. Likewise, Linear functions and Exponential functions both are functions which for each x-axis, there is exactly only one y-axis number.

exponential functions have an asymptote making it appear like there is an x-intercept but it does not meet it at a finite distance.

On the other hand, in a linear equation, there is never an exponent in the function.

A linear function is essentially adding your slope many times, and an exponential function is essentially multiplying your slope.

an Exponential function is more like a hill that gets steeper and steeper as it gets taller in length (so basically a curve).

In the long run exponential equations will always outpace linear growth.

Okay, I might have gone overboard in quoting from my students’ essays, but there’s a ton to think about here! I learned so much about what my students really understood. One particularly interesting misconception involved quadratic functions. We’d moved onto quadratics by the time the essay was due, and I noticed that several students confused exponential and quadratic functions. In one or two cases, everything non-linear was considered exponential. Far from being discouraged by these (and other) misconceptions, however, I found myself empowered to help students develop more robust understandings of the material. Maybe it’s easier to correct procedural mistakes, but it feels so much more meaningful to help students better differentiate between two related concepts and to refine their explanations of the similarities and differences.

So, I think the essay worked. I learned more about what my students knew. My students learned more about linear and exponential functions, and they got an opportunity to engage in mathematical discourse using a different medium. This was a powerful experience for me. I’m toying with the idea of asking my students to write one essay each quarter. Perhaps that’s a bit much, but if I can develop meaningful prompts, then why not do it? I really think writing in math class offers some great possibilities for enhancing learning, and if nothing else, it will allow for the creativity and humor seen in amazing quotes like these:

Have you ever heard of a linear or exponential functions? Well if you haven’t what are you doing with your life?!?!?!

I will preface this paragraph with a disclaimer: truthfully, there are not many similarities between linear and exponential functions, but regardless I will present those that I have knowledge of.

Functions are mathematical concepts that are a mitochondria for the algebra cell.