# EMAT 6680 Assignment 3

#### Robyn Bryant and Kaycie Maddox

The attached 4-page paper is the start of an article that might appear in a journal such as the Mathematics Teacher -- the audience being mathematics teachers who might use some of the ideas for instruction.

It is a start; incomplete, unclear, maybe in error; maybe glossing over significant points and stressing some obvious or trivial points.

Sign on as a co-author.
Rewrite and complete the article. This means you must come to grips with whatever points are to be essential, what to add, what to delete, and what to edit. The "different" approaches to this topic are really in the graphs in the xb, xc, or xa planes. You might want to examine a bunch of these before trying to re-write.

## Some Different Ways to Examine

### by James W. Wilson,Kaycie Maddox and Robyn Bryant 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 -2 < 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

.

As you can see from the graph below, the locus of the vertices of the set of parabolas is itself a parabola. Upon calculating this value for each of the graphs pictured below by using the first derivative of the functions, the vertices are:

All of these points are on the same parabola, the green concave-down one. Since the two zeroes of this parabola are x=1 and x=-1, we can find the factors of the parabola to be (x-1) and (x+1). In addition, we must account for the downward turn of the graph by multiplying it by -1. Therefore, our function of the locus of the vertices of the set of parabolas is:

which is further translated as:

Upon inspection, you will find that each of the above stated vertex points is on this parabola.

### Graphs in the xb plane.

Consider again the equation

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

Notice it is a hyperbola with asymptotes of x=0 and y=-x. It behaves as a graph of y=-1/x does.

If we take any particular value of b, say b = 5, 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.

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

In this case, we find the curve to be that of a hyperbola with asymptotes again of x=0 and y=-x. But this time, the curve acts like y=1/x. Now we can see that for each value of b we select, we can find one negative root and one positive root all along the curve.

### Graphs in the xc plane.

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.

Send e-mail to jwilson@coe.uga.edu
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