The Department of Mathematics Education
EMT 668 - Assignment 3

Some Different Ways to Examine


James W. Wilson and Kimberly Bennekin
University of Georgia

It has now become a rather standard exercise, with availble 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 set

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. The parabola always passes through the same point on the y-axis ( the point (0,1) with this equation). For b < -2 the parabola will intersect the x-axis in two points with positive x values (i.e. the original equation will have two real roots, both positive). For b = -2, the parabola is tangent to the x-axis and so the original equation has one real and positive root at the point of tangency. For -1 < b < 2, the parabola does not intersect the x-axis -- the original equation has no real roots. Similarly for b = 2 the parabola is tangent to the x-axis (one real negative root) and for b > 2, the parabola intersets the x-axis twice to show two negative real roots for each b.

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


The vertex of a parabola can be obtained from the vertex formula: .
For the integers b within the interval [-3,3], we create the following functions:

Their corresponding verticies are:

Looking at the graphs , we can see that these points (the locus) create another parabola with a vertex at (0, 1).

If this is true, we can find the equation of a parabola in which these points are solutions. The standard form for the equation of a parabola is


Where (h, k) is the vertex. This yeilds the equation:


We can find a by using any other point on the parabola. Substitute the point into the equation.

Therefore , the equation of the parabola is:

(This equation can be checked by substituting the other points into the equation.)

Before we generalize, let us consider another set of parabolas,


for b within the interval [-3, 3]. We create the following set of equations:

The overlay of their graphs create the following picture.

These parabolas have the following verticies, respectively:

We can use these points and find the locus to be the quadratic equation: .
It seems that the vertex of the locus will always be the y-intercept of the graph of

, where c is a constant.

Consider again the equation

If we solve for b, we get the relation:

Now graph this relation in the xb plane. We get the following graph.

If we take any particular value of b, say b = 3, and overlay this equation on the graph we add a line parallel to the x-axis. If it 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. 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 positive real root when b = -2, and two positive real roots when b < -2. We can easily see the reason behind this by looking at the discriminant of the original equation:


The discriminant is: , which simplifies to .
If the discriminant is postitive, there are two distinct real roots. If it equals zero, there is one repeated real root. If it is negative, there are no real roots. When we solve the relations

we see why there are two negative real roots when b > 2, one negative real root when b = 2, no real roots for -2 < b < 2, One positive real root when b = -2, and two positive real roots when b < -2.

Consider the case when c = - 1 rather than + 1.

In the following example the equation

is considered. 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. In the graph, the graph of c = 1 is shown. The equation

will have two negative roots -- approximately -0.2 and -4.8.

There is one value of c where the equation will have only 1 real root -- at c = 6.25. For c > 6.25 the equation will have no real roots and for c < 6.25 the equation will have two roots, both negative for 0 < c < 6.25, one negative and one 0 when c = 0 and one negative and one positive when c < 0.

Return to Class Page