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Need help with parabolas


New member
Jul 28, 2013
I've encountered a question that I need help understanding the answer to.

The question is:
What is the equation of the axis of symmetry of the parabola given by the equation y=6(x+1)(x-5)?

Now, I know this quadratic equation is in intercept form, and I know that the formula for finding the axis of symmetry for this is (p+q)/2

Which means that it's (1-5)/2
then -4/2
Which equals -2. But it says that the correct answer is 2, not -2.
I'm confused, is my formula wrong? Please help.


Well-known member
MHB Math Helper
Jan 26, 2012
[JUSTIFY]That's because if $(x - a)$ is a factor of your equation, then the intercept is at $x = a$, not $x = -a$. So in fact your parabola has intercepts $x = -1$ and $x = 5$, and so you would get $\frac{-1 + 5}{2} = \frac{4}{2} = 2$ as expected. The formula itself is correct.

As an example, try plotting $y = x - 5$, and you'll see it intercepts the x-axis at $x = 5$, and not $x = -5$. In the same way, try plotting your parabola and see where it intercepts the x-axis.[/JUSTIFY]


New member
Jul 28, 2013
Ah, I forgot the rule for the x intercept. It's different when looking at a graph than it is in algebraic form. You've definitely helped me, thanks a bunch!