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I received a problem in my math class the other day that left me stumped. The problem went something like this.

Mr. Lots-O-Cash would like to order a parabola that passes through the points $(-4, 0)$, and $(2, 0)$. Create an equation for this so you can pass the information onto the manufacturing department.

I'm wondering, how can I find the equation for this parabola. It seems like to little info to find one. Please don't give me an answer, just a way to solve it.

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    $\begingroup$ There won't be a unique solution. Think about what the $x$-intercepts tell you about the linear factors of the quadratic expression involved. $\endgroup$ Jan 13, 2015 at 18:58
  • $\begingroup$ $y=c(x+4)(x-2)$ for some $c\neq 0$. $\endgroup$
    – Bernard
    Jan 13, 2015 at 19:01

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The general form of a parabola/quadratic is:

$$y=ax^2+bx+c$$

You should be able to construct a pair of simultaneous equations using the two points and solve for $a,b,c$.

Another method is to consider:

$$y=\lambda(x-\alpha)(x-\beta)$$

Where $\alpha , \beta$ are the roots of the parabola.

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  • $\begingroup$ What is the value of $\lambda$ here? $\endgroup$ Jan 13, 2015 at 19:24
  • $\begingroup$ Does the value of $\lamda$ affect the values of the roots? i.e. If you sub the points into the second equation, does it matter what $\lamda$ is, apart from the fact that it can't be 0? $\endgroup$ Jan 13, 2015 at 19:28

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