Effect of Coefficient of x^2 on Parabola Shape

This page and activity authored by James Rutledge, St. Petersburg College. It makes use of an applet created by Larry Green, Lake Tahoe Community College, http://www.ltcconline.net/greenl/java/index.html
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This material was originally developed through Merlot
as part of its collaboration with the SERC Pedagogic Service.

Initial Publication Date: August 19, 2010


This classroom activity presents College Algebra students with a ConcepTest, a Question of the Day, and a Write-pair-share activity concerning the effect of the coefficient of x^2 on the shape of a parabola where b and c are arbitrarily fixed values in f(x)=ax^2+bx+c.

The results are quite revealing and show that while students may have learned that the coefficient of x^2 affects the shape of a parabola (i.e., when a>0, the parabola opens upward, and when a<0, the parabola opens downward), most do not understand the reason why. The fact that the term with the highest degree is the dominant term in a polynomial is generally unfamiliar to them.

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Learning Goals

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

Context for Use

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).

Description and Teaching Materials

Activity Description and Teaching Materials

  • Instructor presents a ConcepTest (Rich Text File 71kB 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^2 on the shape of the parabola. 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 65kB 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 92kB 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^2 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 behavior of the polynomial for large values of a and for large values of x, both positive and negative. (~10 minutes)

Teaching Notes and Tips

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. 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.

References and Resources

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
MERLOT description of this resource