Monthly Archives: April 2016

Exploring Functions (and Non-Functions) Interactively

Heeding Dan's encouragement to step things up in his NCTM talk, I revisited an introduction to functions activity that I put together three years ago. The idea is to get students to make observations about inputs and outputs and use the 'notice and wonder' parlance from the Math Forum to prompt conversations about these ideas.

I rewrote the activity with some deliberate changes and webified it to make it easy to access and share - you can find it here:

Screen Shot 2016-04-29 at 9.55.09 AM

The activity has a few elements that I want to highlight with the hope that you might consider (a) trying the activity with your students or (b) downloading the code for the activity, tweaking it, and then re-sharing it with your enhancements.

Students go through the modeling cycle multiple times.

The activity begs students to take a playful approach. Change the input value and watch the output. Predict what's going to happen and see if your mental model is correct. Then do the next one, and the next.

Arithmetic isn't necessarily a prerequisite.

Some students were actually more puzzled by the functions that took text inputs. They experimented nevertheless to figure out what was happening, and some noticed that the pattern worked for numbers too.

Controversy is built in.

Students working on Functions 5 and 6 saw nothing weird happening when they worked alone. When they then went to share their answers with classmates, the latter function started some really interesting interactions between students trying to figure out who was wrong.

Students of different levels all succeeded and all struggled at some point.

One student zipped through the arithmetic exercises and then got stuck figuring out Function 3 or 7. Some of the weaker students jumped around and got Functions 1 and 4 and 8, which is enough to get in the game of finding patterns and drawing conclusions. A higher level student experimented with Function 7 to find that there was a well defined range for the outputs - random, but with limitations.

The need for definitions came out of the activity, not the other way around.

Students felt the need to clearly define the behavior of Functions 6 and 7 as being different than the others in a fundamental way. Definitions for relations and functions weren't huge cognitive jumps for students since there was a recently established context. It's also important to notice that the definition for relations that aren't functions has to be more than just the lack of a pattern. Function 6 helps with this.

Many of the CCSS standards for mathematical practice are embedded within. are some of the high school standards for functions.

If you try this with students, let me know how it goes.

Technical Details:

If you want to try this yourself, you can download the code from Github here:

I did this also as an attempt to whip together something using the React JS library which I've been learning recently. It makes for a really nice interface for building this type of interactivity into a webpage. There will be more, so stay tuned.

The React components for the eight functions are in lines 86-102 of the index.html file. The function definitions used by each component are defined toward the bottom of the code in that file. You could change these around using Javascript to make these functions fit with your vision of this activity for students. The file is self contained, so you share just the HTML file you change with students, the page will function correctly.

Happy coding!

The Incredible Growing Bricks

I put together this three-act activity two years ago, and decided to include it in the playlist for this year's Math 9 course. The students got right to work in figuring out the total mass of the three bricks together.

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This time, I circulated the actual bricks among the students as they worked. I opted not to do this two years ago because I wanted to force them to use the dimensions in the image above to find their answers. The result was that some students chose to make the measurements themselves rather than use the image. This yielded some great interactions between students asking if the bricks were proportional to each other, and those assuming they were proportional. There were some excellent examples of strong explanations involving proportional reasoning among the student work, as well as typical examples of misconceptions, such as the mass being proportional to the scale factor between the sides.

I also did something new with this modeling task and asked students to predict their uncertainty. Often times, students see that they were close to the actual answer revealed in the third act (but not exactly equal), and subsequently classify their answers as wrong. The uncertainties allow more flexibility in this regard. It also revealed some misunderstanding of the relationship between uncertainty and reporting answers that wasn't unexpected: one student gave 16.895 grams, with an uncertainty of plus or minus 0.1 grams. This is a frequent issue in science classes, but not something I've addressed with mathematics students in the past.

Trigonometric Graphs and the Box Method

Graphing trigonometric functions is a crucial skill, but there is a lot of reasoning involved in the process. I learned the method that follows from a colleague in my first years of teaching in the Bronx, and used it later on when I actually taught Algebra 2 and PreCalculus.

Here's an example for f(x) = 2 cos(2x) + 3:


Students first (lightly) draw a box that is one period wide, twice the amplitude tall, and centered vertically on the midline of the sinusoid. The process of doing this isolates the reasoning about transformations of the function from the actual drawing of the graph, which also takes some skill. I usually ask students to state the period, amplitude, and average value anyway, but this method implicitly requires them to find these quantities anyway. We use the language of the amplitude, period, and average value to describe this box and the transformations  of the parent function.

Once the box is in the right place, then we can focus on the graphing details. Is it a cosine or sine? How does the graph of each fit into the box we have drawn? Where does the curve cross the midline? This conversation is separate from the location and dimension conversations, and this is a good thing. The shape of the curve merits a separate line of reasoning, and encouraging that separation through this method reduces the cognitive demand along the way. I have also seen that keeping this shape conversation separate has reduced the quantity of the pointy sawtooth graphs that students inevitably produce.

I have considered doing this for other parent functions, but haven't been convinced of the potential payoff yet compared with the perfect fit thus far of the trigonometric family of functions. Thoughts?