Using Roots to Generate an Equation


Problem Statement
A parabola crosses the x-axis at x = 2 and x = -1. Find an equation of the parabola.


Find an equation of a parabola that crosses the x-axis at x = a and x = b.


Plans to Solve/Investigate the Problem

Since we know two points on the x axis of the parabola, we can use the graphing calculator program to show us the equation when the points are plotted.


Investigation/Exploration of the Problem

All parabolas of the form y = ax2 are reflected symmetrically over the y axis. Therefore, we know that we need a formula that will cross the x axis at 2 and -1. When using the formula (x + 1)(x 2) = 0 where (x + 1) = A and (x 2) = B, we know that either A or B must be equal to zero to make the equation true. If we solve for x in x+1 = 0, we know that x = -1 and that in x-2 = 0, x = 2. These two points are where the parabola crosses the x axis.


In general, this formula is (x-a)(x-b) = 0. By multiplying this with FOIL, we find x2 bx-ax+ab =0. That means x2 bx ax + ab = 0 and x2 (a+b) x + ab =0. Therefore a= -1 and b=2. Substituting this in the formula x 2 x 2 = y, we can graph this and see that the parabola intersects the x axis at -1 and 2. These points are the roots of the equation.



Author & Contact
Insert name and contact information.
Jill Jackson and Shirley Crawford

jjackson@rockdale.k12.ga.us and scrawford@rockdale.k12.ga.us