Effect of Coefficient of x on Parabola Vertex (b < 0)

To enable students to:

• develop their understanding of the effects of coefficients in quadratic functions
• exercise their mathematical intuition and verify it via appropriate calculations
• exercise the skills of algebraic and mathematical analysis

This activity can be carried out in either a small College Algebra class or a large lecture setting anytime during or after the topic of graphing quadratic functions has been covered.

The activity is comprised of four segments: ConcepTest, Question of the Day, Write-pair-share exercise, and conclusion involving the use of a demonstration applet. The time required for the entire activity is approximately 40 minutes but fewer segments can be offered as a shorter alternative (see Activity Description below for individual segment times).

Activity Description and Teaching Materials

• Instructor presents a ConcepTest (Rich Text File 75kB Aug18 10) in the form of a straw poll (either show-of-hands or written) concerning the effect of increasing the value of the coefficient of x on the position of the vertex. 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 72kB Aug18 10) and students work in pairs to share and explain their reasoning in written form. (~10 minutes)
• Lastly, students are asked to carry out the appropriate calculations in a write-pair-share activity (Rich Text File 135kB Aug18 10) to verify their conjectures. (~15 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 coefficient of x in a quadratic function. http://www.ltcconline.net/greenl/java/Other/MovingGraph/MovingGraph.html
  In addition, the instructor ensures that students understand the algebraic and mathematical analysis involving the expanded form of the formula for the vertex by clearly explaining how the coefficient affects each of the x- and y-coordinates of the vertex. (~10 minutes)

Student understanding of the role of coefficients in polynomial functions is limited at best. Most have not considered the vertex in terms of the values of a, b and c, and many have difficulty in analyzing the effects of changing the values of these coefficients. Working with negative values can present a challenge. This activity is designed to help them develop a better grasp of the role of coefficients.

I generally give the students a grade for participation in this activity and sometimes grade their final calculations concerning the work required.

Larry Green's Applet Page-Parameters and Graphs
This applet allows the user to change the values of the coefficients of a quadratic 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