Forming a sequence inquiry

The prompt

Mathematical inquiry processes: Explore; generate examples and counter-examples; find sets that satisfy conditions. Conceptual field of inquiry: Position-to-term rules; algebraic expressions; graphing a solution set.

The prompt is suitable for older students in secondary school and has led to an inquiry that combines mutually-supporting inductive and deductive reasoning. On first seeing the prompt, classes usually interpret the sequence as arithmetic, although it is advisable for the teacher to establish this constraint if it does not arise in discussion. Students' initial questions and comments about the prompt have been:

Often the solutions (6, 2) and (4, 4) arise early in the inquiry. Whether they do or not, students often decide to explore or "find more examples" when given a choice of the regulatory cards

The teacher can plot the points on a graph as the class finds more pairs of numbers that satisfy the prompt. This can lead to speculation about how the curve develops, which, in turn, enables students to narrow their search to a range of y values for a specific value of x. For example, when x = 5 the value of y must lie between 2 and 4 given that (6, 2) and (4, 4) satisfy the conditions in the prompt.


The prompt was inspired by a problem posted on X (twitter) by Steve Leinwand. His problem was as follows: "The sum, difference, product and quotient of two numbers are the first four terms of an arithmetic sequence. What is the value of the fifth term?" Kier Tipple (an Assistant Headteacher in Brighton, UK) has subsequently worked on the problem and his solution can be viewed here.

Phases of inquiry

The illustration shows the initial questions and observations about the prompt from a year 10 class with high prior attainment in mathematics. 

The majority of students chose to Work as a table when required to select a regulatory card

An exploratory phase of the inquiry led to some of the results in the table:

By the end of the first 100-minute lesson, students had developed three distinct representations: numerical, graphical and algebraic. 

At the end of the lesson, the teacher invited students to summarise each approach on the board (see illustration). 

The class noted that the two formulae giving x in terms of y - one induced from a numerical approach and the other deduced from an algebraic approach - were different.

The second phase of the inquiry (lasting for a 50-minute lesson) was directed by the teacher to ensure all students understood the mathematical content of the first phase. It started with the teacher stipulating that x > y for the purposes of the inquiry. Students who found x = y/(y-3) from numerical examples had assumed y > x.

Some students then set out to show that y = x/(x-3) is equivalent to x = 3y/(y-1) by substituting in the value of y in terms of x from the first formula into the second one. Other students substituted negative values of y into the formula and drew the graph of x = 3y/(y-1) with asymptotes at y = 1 and x = 3.

The third (and final) phase, which lasted 100 minutes, saw students decide on which pathway they would like to develop from the initial list of questions and observations. 

Most of the class continued to select the regulatory card Work as a table

The inquiries involved (1) changing the order of difference, sum and product; (2) attempting to connect the nth term of the sequence to the pair of numbers; and (3) attempting to generate quadratic sequences (using the quotient as well).

Lines of inquiry

1. An algebraic representation

Students, under the guidance of the teacher or independently, have developed an equation giving the relationship between the two numbers. Once students have y in terms of x or x in terms of y, they go on to substitute values for x or y into their equation to find more pairs of numbers that satisfy the prompt.

2. A graphical representation 

If the two numbers are considered as a coordinate pair (x, y) with x ≥ y, then students can plot a graph. Further inquiry of the asymptotes at x = 3 and y = 1 has been sparked by comparing the algebraic equation and graph together. 

3. Connecting the two numbers to the nth term

One popular line of inquiry in the classroom involves attempting to connect the two numbers to the nth term of the sequence. A table helps students to organise their results and can lead to a recognition of the coefficient of n as 2y. However, the constant is more difficult to identify from the table. As the first term (n = 1) of the sequence is created by x - y, then x - y = 2y + c and, it follows, c = x - 3y.

Further lines of inquiry

Even though students have been involved in substantial mathematical thinking up to this point, the prompt has the potential for a richer inquiry. To encourage further questioning, the inquiry teacher could use the strategy laid out in The Art of Problem Posing (see the post on questioning in inquiry):

(Level I) List the attributes

(Level II) Modify the attributes by asking "What-if-not?"

(Level III) Pose questions

(Level IV) Analyse the questions

The inquiries that now follow tend to involve individuals, pairs or groups choosing to answer one of the questions above with some form of feedback to the rest of the class.

Students' questions

These are the questions from Caitriona Martin's year 10 class on first seeing the prompt.

Recurrence relation

One notable result came out of the sequence that was generated from 5 and 10. Using the first three terms (5, 15, 50), one student generated the fourth term by using a term-to-term rule. Caitriona then helped to express the rule formally as a recurrence relation.

July 2014

Students' inquiry reports