2.5: Finding Factors from Roots
( \newcommand{\kernel}{\mathrm{null}\,}\)
One method of solving equations involves finding the factors of the polynomial expression in the equation and then setting each factor equal to zero.
\begin{array}{c}
x^{2}+8 x+15=0 \\
(x+5)(x+3)=0 \\
x+5=0 \quad x+3=0 \\
x=-5 \quad x=-3
\end{array}
In this process, the reasoning is that if (x+5) times (x+3) equals zero, then one of those expressions must be equal to zero. In setting them equal to zero, we find the solutions of x=-5,-3 . Plugging them back into the factored expression we see the following:
(-5+5)(-5+3)=0 *-2=0
and
(-3+5)(-3+3)=2 * 0=0
This process works in reverse as well. In other words, if we know a root of the function, we can find factors for the expression.
Example
Find a quadratic equation that has roots of -2 and +3
\begin{array}{cc}
x=-2 & x=3 \\
x+2=0 & x-3=0 \\
(x+2)(x-3)=0 \\
x^{2}-x-6=0
\end{array}
Roots that are fractions are a little trickier, but really no more difficult:
Example
Find a quadratic equation that has roots of -5 and \frac{2}{3}
\begin{array}{cc}
x=-5 & x=\frac{2}{3} \\
x+5=0 & 3 x=2 \\
x+5=0 & 3 x-2=0 \\
(x+5)(3 x-2)=0 \\
3 x^{2}+13 x-10=0
\end{array}
Exercises 2.5
Find a quadratic equation that has the indicated roots.
1) \quad 4,-1
2) \quad -2,7
3) \quad \frac{3}{2}, 1
4) \quad-\frac{1}{5}, \frac{2}{3}
5) \quad \frac{1}{3}, 3
6) \quad-4, \frac{2}{5}
7) \quad \frac{1}{2},-\frac{7}{2}
8) \quad-1, \frac{3}{5}
9) \quad-\frac{2}{3},-3
10) \quad-\frac{2}{3},-\frac{3}{4}
11) \quad-\frac{5}{2}, 3
12) \quad-6,-2