Circle – Square Problem (Stolen from Dan Meyer’s site)

We are only three (well, really two and a half) days from spring break. Spring itself comes late in NE PA but spring break comes early for us. We are in the thicket of infinite series in Calc BC and I wanted to take the last three days on a tour of an old AP Free Response section. I wanted to send them on their break with the full realization that they know almost everything that they’ll need to know in May. So, yesterday I ran off copies of the two calculator questions from an AP test along with the grading rubric. Figured it would be a good working day while I listened in and roamed a bit. Then at 6 this morning I read Dan Meyer’s newest blog post and immediately decided to scrap my plans. He presented the following problem

Given an arbitrary point P on a line segment AB, let AP form the perimeter of a square and PB form the circumference of a circle. Find P such that the area of the square and circle are equal.

and, as usual, presented a challenge to his readers.

What can you do with this? How can you improve the task?

I’m not sure how much I improved the task, but I did make some decisions about what to do with it. The first decision I made was to hand my students some blank paper and simply read the question to them. I read it carefully twice. I did not want to visualize it for them, I wanted to see what they would do with this. I was surprised that most of them did their best to jot down the exact words that I said.  (This was in my quiet, small morning group – I am just getting ready to meet with my afternoon group) Almost all of them proceeded to draw a line segment and label a point P pretty near the middle. One student identified the two partitions as x and y. Very algebraic! My more engaged (and much larger) second class responded almost unanimously by drawing a segment. They hardly bothered with the words at all. One student had a segment with a square imposed on one side and a circle imposed on the other. Nice.

Both classes were comfortable (being BC Calculus kids, I expected them to be) with some general statements of the area either interns of a variable or in terms of segment notation. Both classes decided that it would be nice to have a default length for the segment AB and they each suggested 1 as the length. This was interesting, I went immediately to 100 as a nice default value. I was thinking in terms of a percentage of length. My first class let me get away with that and we reached a solution to the quadratic pretty quickly. My second class was insistent that 1 was a better choice. Of course, the critical balance value is found regardless. By the time the afternoon class had met the hive mind was working full bore on this problem. I shared Dan Anderson’s terrific Desmos demonstration and I also showed the the Math Hombre’s GeoGebra sketch of the problem.

As per Dan’s suggestion on twitter, I asked my students to try and optimize the situation. Both classes were very quick to conclude that the largest area is the trivial case of the entire segment being used to make the circle. When we graphed the function that represented the sum of the areas it looked tantalizingly close to the equal area point being the minimum. Sadly, this was not to be. We also discussed another scenario proposed on twitter and that was to have different regular shapes, not just a circle and a square.

I was proud of how open my students were to exploring this open-ended question. I was impressed that they were such careful listeners when I presented the problem to them verbally. Many of them are not note-takers, but they were willing to dive in and play on paper with this problem.

I am left with a some ideas/issues to ponder.

  1. I need to figure out a way to find time at the beginning of next year to give my students the opportunity to familiarize themselves with Desmos and GeoGebra. I wonder how I can structure this appropriately. When they see the dynamic visualizations the conversation opens up.
  2. By my standards, I think I did a pretty good job of getting out of the way today. I still started too many of the conversations, but I let the questions percolate from them. I need to find some sort of mantra or something to remind myself to be more quiet and take more time to let the questions arise.
  3. As with many class conversations, the pace was dictated by a few students who are more eager to share ideas and ask questions. I need to work on respecting this while also creating a buffer for those who take a few more moments to ponder.

So happy I scrapped my plan of canned AP FR questions today. I hope that the students are happy about this as well.