

Exploration of Two Growth Models: A Unit for the Curriculum Course EMAT 4500/6500 Connections in
Secondary Mathematics
Heide G. Wiegel Mathematics Education The University of Georgia
Table of Contents 

Introduction 
As part of the course EMAT 4500/6500 Connections in Secondary Mathematics, groups of
preservice teachers investigate a unit from a contemporary, standardsbased high school
mathematics textbook. Two textbook series are available to the preservice teachers:
Interactive Mathematics Program [IMP] (Fendel, Resek, Alper & Fraser, 1997), published by
Key Curriculum Press, and Contemporary Mathematics in Context [CorePlus], published by the
EveryDay Learning Corporation. The requirements for the group projects are twofold: (a) preservice
teachers have to write individual project reports, and (b) each group has to give a
presentation of their unit to the class.
Individual reports should include:
 overview of the unit;
 writeups of a representative selection of worked problems in the unit;
 a summary of mathematical and nonmathematical concepts introduced or reviewed in the unit;
 a description and evaluation of the group's working arrangements;
 an affective and conceptual evaluation of the unit (a) with the eyes of the (future) teacher, and (b) with the eyes of the
(former) high school student;
 two questions for the final exam (final exam is designed by the instructor on the basis of the group suggestions; groups are encouraged to share their exam suggestions);
 material (e.g., overheads) of the group presentation.
The individual reports are evaluated by the instructor.
Group presentations should include:
 a brief overview of the textbook for the grade that contains the unit;
 a more extensive overview of the unit and the mathematical concepts in the unit;
 at least one activity that involves the class and highlights the essence of the unit;
 a homework assignment that reinforces concepts of the unit and of the class activity and is comparable to the final exam questions; the homework assignments are graded by the group.
The group presentations are evaluated by all class members, including the presenters
and the instructor (see Appendices IA and IB). Groups have access to the teacher's edition of their unit after they
have worked through the unit. During the semester, group presentations lasted usually one whole class session (once a week). A different course schedule will require a different
organization of the presentations.
Reflecting on the course and on the group presentations and individual reports, I decided
to present—in a revised course—one unit myself during the first part of
the course. Thus, preservice teachers will have a model they can use or modify for their
own group presentations. I chose the unit Small World, Isn't it? from the third year of
the IMP series. I plan to present the unit in one class session and to extend it in a second session.

Small World, Isn't It? 
Each IMP unit starts with the unit question that usually is ambiguous and ill defined. The
first task for high school students is to interpret the unit question, to define the ill–defined
conditions, to think about the issues involved, and to come up with a preliminary list of
information, skills, and mathematical tools they need to acquire in order to answer the unit question.
As the students work through the unit they collect information and encounter and study the necessary
mathematical concepts and skills. Not all classwork or homework investigations are thematically
connected to the unit question. Throughout the unit, however, the class returns to the unit question
and takes stock of the progress made. The unit concludes with the solution of the unit
question and with the compilation of portfolios in which students collect and submit their best work. 
The Unit Question 
The central question of the SmallWorld unit is initially presented as
If population growth continues according to its current pattern,
how long will it be until people are squashed up against each other? (Teacher's Guide, (see Fendel, Resek, Alper & Fraser, 1999, p. xxvii)).
A table with estimated U.S. population data from 1650 to 1995 and information about the total
surface area and the percentage of land (see Overhead 2) are provided.
Based on the initial student work and after a definition of squashed up against
each other as one square foot per person, the unit question is formulated as
Based on the data in “Small World, Isn't it?” how long will it take until the
population reaches 1.6 * 10^{15} people? (Teacher's Guide,(see Fendel, Resek, Alper & Fraser, 1999, p. 12)).

Outline of Unit (Teacher's Guide, p. xxix) 
 Days 12: Introduction to the unit problem;
 Days 35: Working with average rates of change;
 Days 610: Developing the concept of slope and working with linear functions;
 Days 1116: Developing the concept of a derivative from several perspectives;
 Days 1718: Examining situations that are modeled by exponential functions;
 Days 1921: Studying the derivatives of exponential functions, leading to the discovery of the “proportionality property”;
 Days 2226: Working with bases and exponents to see that any base can be used to represent a given exponential function, then searching for the “best” base;
 Days 2729: Developing the connection between compound interest and the special base e;
 Days 3032: Solving the central unit problem, using curvefitting techniques, and compiling unit portfolios;
 Days 3334: Unit assessments and summing up.

Concepts and Skills (Teacher's Guide, pp. xxxxxxi) 
Rate of change
 Evaluating average rate of change in terms of the coordinates of points on a graph
 Understanding the relationship between the rate of change in terms of a function and the appearance of its graph
 Realizing that, other things being equal, the rate of change in population is proportional to the population
Slope and linear functions
 Developing an algebraic definition of slope
 Proving, using similarity, that lines have a constant slope
 Understanding the significance of a negative slope in terms of the appearance of a graph
 Seeing that the slope of a line is equal to the coefficient of x in the algebraic representation of the line
 Using slope to develop equations of straight lines
Derivatives
 Developing the concept of the derivative of a function of a point
 Seeing that the derivative of a function at a point is the slope of the tangent at that point
 Finding numerical estimates for the derivatives of functions at specific points
 Working with the derivative of a function as a function in itself
 Realizing that for functions of the form y = b^{x}, the derivative at each point of the graph is proportional to the yvalue at that point
Exponential and logarithmic functions
 Using exponential functions to model reallife situations
 Strengthening understanding of logarithms
 Reviewing and applying the principles that a^{ b} * a^{ c} = a^{ b+c} and (a^{ b})^{c} = a^{ b*c}
 Understanding and using the fact that a^{log ab} = b
 Discovering that any exponential function can be expressed using any positive number other than 1 as a base
 Learning the meaning of the terms natural logarithm and common logarithm
 Using an exponential function to fit a curve to numerical data
The number e and compound interest
 Estimating the value b for which the function y = b^{x} has a derivative at each point on its graph equal to the yvalue at that point
 Developing and using a formula for compound interest
 Seeing that expressions of the form (1 + 1/n)^{n} have a limiting value, called e
 Learning that the limiting value e is the same as the special base for exponential functions

Class Organization 
The IMP material uses three primary working arrangements: Small–group work in class, homework, and
whole–class discussions. The teacher is not lecturing unless there are technical terms to be
introduced or reviewed. Most of the conceptual work occurs during the small–group sessions when
students form conjectures, look for patterns, or prepare a class presentation. Homework assignments
either follow up on group work and class discussions or serve as preparations for the next lesson.
The homework assignments consist of a few conceptual problems rather than masses of
drill–and–practice exercises.
Whole class discussions are a major part of the lessons. The discussions serve as forums in
which students present results from homework and smallgroup work, clarify their understandings of the
concepts involved, and formulate new questions. The teacher serves a moderator rather than as presenter.

Class Activities for PreService Teachers 
For my presentation of the unit to preservice teachers, I am assuming an instructional format in
which the course meets for about 3 hours one evening per week. Given the nature and time limits of
the class presentation, some lecture (e.g., overview of the unit) will be necessary. However, any
kind of lecture needs to be structured by short class activities in order to counteract the late
time of the day and the length of the class session. I intend to include two miniactivities and
two longer activities, all taken from the student book of the IMP unit.
Miniactivity 1: Classwork 1 "Small World, Isn't It?" will be attached to the introduction of
the unit question (1015 minutes individual or small group work, according to individual students'
choices; followed by a collection of ideas on the overhead; see Overheads 2 and 3).
Miniactivity 2: Modification of Homework 16, "What's It All About?" (see
attached copy of student page, Overhead 5); for the students in a high school class, this homework is a summary of what they have learned about derivatives.
For the secondary preservice teachers, the miniactivity is a review of college work that might have already retreated to the back of their minds. The miniactivity
will serve to establish a common basis for the remainder of the presentation.
Activity 3: Modification of Classwork "Slippery Slopes" (see Overhead 7). In
this activity high school students explore the derivative of exponential functions and look for a pattern
of how the derivative might relate to the xvalue (independent variable), the yvalue (dependent variable),
or both. I chose this activity for the work with the preservice teachers because it highlights the
experimental approach taken in the IMP materials: students collect data and look for patterns in the
data; abstractions are built on a firm ground of experiences. The approach differs fundamentally from
the more theoretical approach usually taken in Calculus courses. High school students will most likely
use the calculator to find the slope of the secant and the ratio of the slope to the value of the
exponential function. Excel (see Appendices IIA, IIB, and IIC) provides an
efficient tool for this exploration,
and preservice teachers can use either tool. (See student pages 1 and 2).
Activity 4: In this activity, the preservice teachers will use Excel
and its many features to explore how the world population might grow in the next 100 or 200 years.
I intend to use a second set of data from the UN 1998 world population
report (http://www.undp.org/popin/) for comparison.
Some differences exist between the two sets of data: (a)
The UN data start with a population estimate in Year 0 whereas the IMP data start at 1650. (b)
The two sets have different estimates for the year 1750. The UN data show 790 million people, the IMP
data show only 694 million. (c) The data for 1990 are closer, but still different (UN—5.27 billion,
IMP—5.29 billion). Based on these differences, we should expect different estimates for the
time it takes to fill up the earth to standing room only. (See student page 3).

Homework 
Problem of the Week [POW] 8: The More, the Merrier? (Essay, see attached copy of student page).

Outline of Session 1 
 Introduction of the unit question and the properties common to IMP unit questions (Teacher presentation [TP]);
 MiniActivity 1 (small groups or individual students; wholeclass summary);
 Overview of unit (TP);
 MiniActivity 2 (whole class);
 Overview of Days 1723: A Model for Growth (TP);
 Activity 3 (small groups; wholeclass summary);
 Overview of Days 2429: The Best Base (TP);
 Activity 4 (computer work; individual or in pairs).
Appendices VIA, VIB, VIC and VII contain some of my own
explorations with the data and different approximating functions. Tables and graphs
in Appendices VIA, VIB, and VIC are based on UN data; tables and
graphs in Appendix VII are based on IMP data.

The Extension: The Logistical Growth Model 
IMP's definition of being squashed up against each other as one square foot per person and the
assignment The More, the Merrier? invite two questions: "How many are too many?" and "Can population
growth go on for ever?" In contrast to the exponential growth model that allows unrestricted growth, the
logistic growth model introduces a limiting value K, the carrying capacity. The logistic growth model is
also called the Verhulst model, named after the Belgian mathematician P. F. Verhulst who developed and
studied the model in the 19th century. Based on the US census data from 1790 to 1840, Verhulst used the
logistic model to predict the US population in 1940. His estimate was off less than 1% . For census
results after 1940, however, the logistic model does not fit the actual US population growth (see
Appendix III and http://www.math.duke.edu/education/ccp/materials/diffeq/logistic/contents.html).

The Logistical Growth Function 
The growth rate in the exponential model is proportional to the population, that is,
f'(t) = r f(t), or dP/dt = rP.
Because the IMP deals with world population data, the proportionality constant r can be interpreted as the
birth rate minus the mortality rate, that is, we do not have to consider other variables such as
immigration or displacement. In the logistic growth model, the proportionality constant r is replaced
by a more complex growth rate:
The function P(t) is given by
It depends on the group of preservice teachers whether or not the development of the function P(t)
is appropriate for the course. However, developing, understanding, and presenting the function
might be a worthwhile task for graduate students.

Development of Closed Form 
The rate of change is given as
Step 1. Separate variables:
Step 2. Change so it can be integrated:
Step 3. Integrate:
Step 4. Exponentiation gives
Step 5. For t = 0, we get
Therefore,
Step 6. Invert the previous equation:
Step 7. Invert again
Step 8. Finally, we get

The Simulation 
Marilyn Stor and William L. Briggs (Stor & Briggs, 1998) introduce the
logistic growth model in the context of an investigation of communicable diseases. They
present a gamelike simulation, a computer simulation, and the development of a recursive
rule. Because EMAT 4500/6500 starts with a set of problems that lend themselves to
recursive–as well as closed–formula representations, an introduction of the
logistic growth model through a
recursive approach is appropriate. The simulation game and its analysis will be at the
heart of the second session. During Fall 1998, Cathryn Brooks, a master student in Mathematics Education,
translated the provided program into PASCAL (see Appendix IV) and developed the computer
simulation. Thus, preservice teachers can experiment with different infection rates. They can also apply the
recursive rule in Stor and Briggs (1998) and use Excel for experimentation. The recursive rule is given as
I_{n+1} = I_{n} + N_{n} = I_{n}
+ aI_{n}(T – I_{n})
I_{n} and I_{n+1} represent the number of infected persons at stages n and n+1,
respectively, N_{n} is the number of people newly infected at stage n, and a is the proportionality
constant. T represents the total population and corresponds to the carrying capacity K in the logistic
population function. The recursive rule assumes no births, deaths, or immigration. However, students
should discuss how these population changes and other factors such as education, immunization,
and recovery might change the infection rate. After the game and its analysis, students will be
given local flu data (from Athens Regional Medical Center, not yet available) and several sets of US
flu data (see Appendix V
and http://www.cdc.gov/ncidod/diseases/flu/fluvirus.htm).
Data will be modeled in Excel, and
preservice teachers will experiment with different values of the proportionality constant
a in order to find the recursive rule for the flu data. The TI83 calculator offers logistic regression.

Homework 
http://www.imdb.com/Charts/usboxarchive
lists earnings of current movies by week. Students will select a movie and investigate
the cumulative revenue function of the movie. 
Outline of Session 2 
 Presentation of results of Activity 4 and discussion of homework (whole class); transition;
 The simulation game
 Introduction
 Preparation
 Game
 Collection of data
 Analysis: How many are infected?
 Graph
 The logistic growth model (TP and class discussion; history, formulas as appropriate);
 Computer/calculator investigation of flu data;
 Extension: Investigation of the fit of the logistic growth function to a set of
world population data projected to 2150 (logistic regression with the TI83 or Excel
experimentation with different values of r).
Appendices VII and VIII show some of my own explorations with the game and flu data.

Sources 
 Fendel, D. Resek, D. Alper, L. & Fraser, S. (1997). Interactive mathematics program (Year 3, pp. 282–387). Berkeley: Key Curriculum Press.
 Fendel, D. Resek, D. Alper, L. & Fraser, S. (1999). Small world isn't it?, Teacher's guide. Berkeley: Key Curriculum Press.


