Exploring Sequences

In Discrete math we are exploring recursive sequences and talking about how to make them explicit. When given a table and a recursive definition, my team of Discrete Math warriors has gotten pretty good at examining first differences, second differences, etc. and relating them back to the degree of an explicit formula. I recognize that some of this is rote, but sometimes skill development looks like that. It was not until I presented the following problem that I realized how rote some of the problem-solving has been. Here is the problem:

Suppose that Hamilton is playing at the Civic Auditorium. The auditorium has only one section for seating. The seats are arranged so that there are 60 seats in the first row, 64 seats in the second row, 68 seats in the third row, etc. So, in each successive row there are four seats more than in the previous row. There are a total of 30 rows in the auditorium.

• How many seats are in the last row?

• How many seats are there in the auditorium?

• The seats are numbered consecutively from left to right, so row two starts with seat 61, row three with seat 125, etc. You purchased a ticket to the play and your seat number is 1500. What row are you in? Where in that row is your seat located?

So, the first part of the problem went reasonably well. They were able to recall that there are 29 steps of equal size to be taken in accounting for row size, but even this was harder than it should have been due to the reflex to create a table. I began to realize that what seems automatic to me, that we are concerned with row number and with accumulated seats, was not automatic to most of my students. They set up a column of row numbers followed by a column of row size. They then arrived at a first difference of 4 for each entry in their third column and they were off on finding a linear function. The linear function is correct for the number of seats in each row, but the rest of the problem depended on them finding an accumulated number of seats. When I set up a table and had row number followed by #of seats in that row and then the total number of seats, the inconsistence with previous visuals was a real problem. Getting my students to focus on the first and second differences in this new third column was a challenge. I know that I did not answer their concerns as clearly as I need to and I have to figure out how to better answer this. Once we established that this is a quadratic relationship we were able to find the coefficients and answer the second question. It took some convincing and looking at some smaller sums along the way, but I think we came to a genuine consensus. Switching over to the third part of the question was a giant hurdle. I did not intend for them to solve a quadratic since they would get an irrational solution. Instead, I hoped for some reasonable guess and check but it became clear that, for too many of them, the ladders of abstraction leading to this part of the problem completely clouded the problem for them. I have faith that this is an interesting question. In the spirit of full disclosure, it is important to note that this is simply a (slightly) modified form of a problem from our publisher’s test bank. What I need to think deeply about are the following questions:

1. How much quadratic function review do I want to do to help set up a meaningful context for these recursive functions? My gut feeling was that I did not want to go into those thickets with these kids. Many of the students in this class are realizing that they can do some mathematical thinking once they removed themselves from thinking that mathematical thought only looks like equation solving.
2. How do I balance the discrete nature of this problem with the inherently continuous point of view that students have regarding quadratic functions?
3. How do I help my students focus on building a table of data that is clear and meaningful? How to focus more clearly and quickly on the pertinent data in the problem?
4. How can I carefully structure a positive class discussion around one in-depth, challenging problem like this in a class where too many of the students have felt defeated by math one too many times? I feel great about the general atmosphere we have created together and I want to keep that while extending their thinking.

Sadly, I have to wait until next year to make this better.