The Department of Mathematics Education

EMT 668 Assignment 3

Some Different Ways to Examine


James W. Wilson and Kelli L. Nipper
University of Georgia

It has now become a rather standard exercise, with available technology, to construct graphs to consider the equation

and to overlay several graphs of

for different values of a, b, or c as the other two are held constant. From these graphs, discussion of the patterns for the roots of

can be followed. For example, if we graph

for b = -3, -2, -1, 0, 1, 2, 3, and overlay the graphs, the following picture is obtained.

We can discuss the "movement" of a parabola as b is changed. Notice that the parabola always passes through the same point on the y-axis (at y=1), but the minimum point of each graph shifts as b changes. This affects the sign and number of roots.

We can determine the vertices of the set of parabolas using the derivative of the equation:

Since the slope of a horizontal tangent line is zero, we can set the derivative equal to zero, and solve for x.

y' = 2ax + b
0= 2ax + b

Now consider the locus of the vertices of the set of parabolas graphed from

We can calculate our y-coordinates using:

Plotting these points shows that the locus is the parabola

The locus of the parabola can be found by substituting

into the equation:

So the locus is the parabola y= -x^2 + 1

In general, for any equation

the locus is the parabola

Consider again the equation

When we graph this relation in the xb plane, we get the following graph:

If we overlay the line b = 3 , we can use this to determine the roots of the parabola. If the line intersects the curve in the xb plane, the intersection points correspond to the roots of the original equation for that value of b. We have the following graph:

For each value of b we select, we get a horizontal line whose intersection correlates with the signs and number of roots for varying values of b. It is clear on a single graph that we get two negative real roots of the original equation when b > 2, one negative real root when b = 2, no real roots for -2 < b < 2, one real root when b = -2, and two positive real roots when b < -2.

Consider the case when b is set constant at 5 and c varies.

In the following example the equation

If the equation is graphed in the xc plane, it is easy to see that the curve will be a parabola. For each value of c considered, its graph will be a line crossing the parabola in 0, 1, or 2 points -- the intersections being at the roots of the orignal equation at that value of c. The graph of c = 1 is shown.

The real roots are the x-values of the intersection of c = 1 and:

The same situation applies when we consider the graph where b and c are constant and a varies:

Graphing the equation in the xa plane, we can find the roots for various values of a.

In general, we can graph in the xa, xb, xc planes to show the connection between geometry and algebra.

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