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 Using Algebra and Discrete Mathematics to Investigate Population Changes in a Trout Pond
Part Two - Numerical Analysis
 Table Graph  Equation
  Make a Conjecture Numerical Analysis Graphical Analysis Symbolic Analysis

Model the Situation

This situation can be modeled by the equation: NEXT = 0.8 NOW + 1000 (start at 3000)

  • Explain what the words NOW and NEXT represent in this equation.

  • Where does the factor of 0.8 come from?

  • Explain why the equation represents this situation.

More formally, the equation can be written as:

A(n + 1) = 0.8 A(n) + 1000, A(0) = 3000

  • Explain what n, A(n), and A(n+1) represent in this equation.

  • Use A(n) and A(n-1) to write another equation that also represents this situation. This equation should begin A(n) = ... Explain why this new equation and the A(n+1) = ... equation above are both accurate representations of this situation.

These equations can be used to create a spreadsheet for this situation.

Now  output.



View a Spreadsheet

If this spreadsheet is blank then  output.

 

Analyze the Spreadsheet

  • Was your conjecture about the long-term population correct?

  • Explain, in mathematical terms and in terms of the fishing pond ecology, why the long-term population computed in the spreadsheet is reasonable.

  • Does the trout population change faster around year 5 or around year 25? How can you tell?

  • Based upon this spreadsheet output, when do you think the fish population will reach 5000?

  • Do you think that the mathematical model ever actually yields 5000? Consider your answer, then click to see for years 80 - 99. Now what do you think? Explain.

Go to the Next Step

What If?

Find out what happens when the assumptions change.

There are three key factors in this problem - the initial population, the annual restocking amount, and the annual population decrease rate.

1. Consider the three questions below. Make conjectures for the answers.

(a) If the initial population doubles, what will happen to the long-term population?
(b) If the annual restocking amount doubles, what will happen to the long-term population?
(c) If the annual population decrease rate doubles, what will happen to the long-term population?

2. Enter new values in the spreadsheet and press "Calculate" to generate new spreadsheet outputs. Use the spreadsheet to answer questions a, b, and c above, and compare the answers to your conjectures. Any surprises? Any patterns? (Note that Growth Factor = 1 - annual population decrease rate)

Enter Values
Initial Population:
Restocking Amount:
Growth Factor:



3. Systematically investigate the effect that other changes in these three factors will have on the long-term population. Keep track of the results of your investigations. Describe any patterns you see.

4. Suppose the annual decrease rate is 20% and the initial population is 3000, as in the original situation. But now suppose you want to change the restocking amount so that the long-term population will be 7500. What restocking amount should you use?

 

 


Next Step

So far you have investigated this situation numerically, by looking for
patterns in the values of A(n).


To get a better understanding of the patterns and why they are occurring,
it is helpful to do a Graphical Analysis.

 

Make a Conjecture Numerical Analysis Graphical Analysis Symbolic Analysis


References and Credits


© 2000 - National Council of Teachers of Mathematics


CD Version last updated: August 8, 2000