# Effect of Proportionality Constant on Exponential Graph (k < 0)

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This page first made public: Aug 19, 2010

This material was originally developed through Merlot as part of its collaboration with the SERC Pedagogic Service.

#### Summary

This classroom activity presents College Algebra students with a ConcepTest and a Question of the Day activity concerning the effect of the proportionality constant, k, on the y-intercept and position of an exponential graph where k<0 and C is an arbitrarily fixed value in f(x)=Ce^(kx).

While students may have learned that the proportionality constant affects the shape of a exponential curve, most do not realize exactly how. Analyzing the effect requires some effort on the students' part but the results are rewarding.

## Learning Goals

To enable students to:

- develop their understanding of the effects of the proportionality constant in an exponential function
- exercise their mathematical intuition and verify it via appropriate calculations
- exercise the skills of algebraic and mathematical analysis

## Context for Use

The activity is comprised of three segments: ConcepTest, Question of the Day and Conclusion, involving the use of a demonstration applet. The time required for the entire activity is approximately 25 minutes but fewer segments can be offered as a shorter alternative (see Activity Description below for individual segment times).

## Description and Teaching Materials

Activity Description and Teaching Materials

- Instructor presents a ConcepTest (Rich Text File 82kB Aug18 10) in the form of a straw poll (either show-of-hands or written) concerning the effect of increasing the proportionality constant, k, on the position of the y-intercept and the position of the graph with regard to quadrants. Each student is asked to make a conjecture and the instructor records the results for the class to see. (~5 minutes)
- After the straw poll, the question becomes the Question of the Day (Rich Text File 79kB Aug18 10) and students work in pairs to share and explain their reasoning in written form. (~10 minutes)
- In conclusion, the instructor presents a classroom demo involving a Java applet that quickly and easily demonstrates the effects of changing the value of the proportionality constant in an exponential function. http://www.ltcconline.net/greenl/java/Other/MovingGraph/MovingGraph.html

In addition, the instructor ensures that students understand the algebraic and mathematical analysis involved and clearly explains the reasons for the effects. (~10 minutes)

## Teaching Notes and Tips

## Assessment

## References and Resources

This applet allows the user to change the initial value and the proportionality constant of an exponential function and immediately see the graphical results.

http://www.ltcconline.net/greenl/java/index.html

http://www.ltcconline.net/greenl/java/Other/MovingGraph/MovingGraph.html

MERLOT description of this resource

http://www.merlot.org/merlot/viewMaterial.htm?id=77279