SITE-A teams: Lesson 19

A quadratic polynomial presents a distinctive "signature" when its graph is drawn; it is a parabola. The size (fat or thin), direction (opening up or opening down), and position on the xy-plane vary but the curve is a pattern that students can recognize. Reference: Lopez, A. (1996) Pattern Matching, Searching and Heuristics in Algebra, Mathematics and Computer Education, 30, 2, 255-266.

Algebra students are often faced with three types of problems dealing with quadratic polynomials:

1. Find the real-valued roots (x-intercepts) of a given quadratic polynomial.
2. Given a quadratic polynomial equal to zero, solve for the real-values of x.
3. Factor a given quadratic polynomial.

All three of these activities are closely related and can be accomplished graphically using the CASIO fx7400G.

Consider the following:

1. Find the roots of      y = x2 + 4 x + 5
2. Solve for x:              x2 + 4 x + 5 = 0
3. Factor                      x2 + 4 x + 5

Turn on the CASIO fx7400G.

Use your cursor arrows to highlight the GRAPH menu and press or just press when at the main menu screen.

In the Y1 slot enter the expression X2 + 4 X + 5 and press

(Make sure no other Y-slot is highlighted, i.e. selected for graphing.)

Press (V-Window)

Press (INIT)

Press

Press (DRAW)

The graph appears on the screen of the calculator (See graph below).

Since the graph does not cross the x-axis, the answers are:

1. there are no roots (x-intercepts).
2. the equation has no solution.
3. the polynomial can not be factored and therefore is irreducible.

Press

Highlight the expression and de-select it by pressing (SEL)

or delete it by pressing (DEL)

Consider the following:

1. Find the roots of     y = x2 - 2 x + 1
2. Solve for x:             x2 - 2 x + 1 = 0
3. Factor                    x2 - 2 x + 1

In the Y2 slot enter the expression X2 - 2 X + 1 and press

(Make sure no other Y-slot is highlighted, i.e. selected for graphing.)

Press (DRAW)

The graph appears on the screen of the calculator (See graph below). Press (Trace) and use the right cursor arrow to move the "cross hairs" to where the graph touches the x-axis. This is at x = 1 and y = 0.

Since the graph touches the x-axis in only one place, the answers are:

1. the real-valued root (x-intercept) is 1.
2. the solution to the equation is x = 1.
3. the polynomial factors into ( x - 1 )2.

Press

Highlight the expression and de-select it by pressing (SEL)

or delete it by pressing (DEL)

Consider the following:

1. Find the roots of     y = - x2 + x + 6
2. Solve for                x: - x2 + x + 6 = 0
3. Factor                    - x2 + x + 6

In the Y3 slot enter the expression - X2 + X + 6 and press

(Make sure no other Y-slot is highlighted, i.e. selected for graphing.)

Press (DRAW)

The graph appears on the screen of the calculator (See graph below). Press (Trace) and use the right cursor arrow to move the "cross hairs" to where the graph touches the x-axis. This is at x = - 2 and y = 0 and at x = 3 and y = 0.

Since the graph crosses the x-axis in two places, the answers are:

1. the real-valued roots (x-intercepts) are -2 and 3.
2. the solutions to the equation are x = -2 and x = 3.
3. the polynomial factors into ( x + 2 )( x - 3 ).

Press

Highlight the expression and de-select it by pressing (SEL) or delete it by pressing (DEL)

Press

Turn off the CASIO fx7400G. (OFF)

Graph of y = x2 + 4 x + 5

Graph of y = x2 - 2 x + 1

Graph of y = - x2 + x + 6