1. ## which quadratic formula do I use

I posted a problem I had with graphing a quadratic equation but im now not sure if im even using the right formular,

the original expression to graph is P = 85X - X^2 - 550

Am I right in changing this to F(x) = -X^2 + 85X -550?

i.e. a=-1, b=85, c=-550

and then I don't now whether to use either

-b/2a, or, -b sqrt(b^2-4ac)/2a

...in order to get the X vertex.

sorry to post the same sort of question but I'm having real trouble getting through this one with such large values for b and c

2. Originally Posted by bobchiba
I posted a problem I had with graphing a quadratic equation but im now not sure if im even using the right formular,

the original expression to graph is P = 85X - X^2 - 550

Am I right in changing this to F(x) = -X^2 + 85X -550?

i.e. a=-1, b=85, c=-550

and then I don't now whether to use either

-b/2a, or, -b sqrt(b^2-4ac)/2a

...in order to get the X vertex.

sorry to post the same sort of question but I'm having real trouble getting through this one with such large values for b and c
Yes you're right to rearrange the order, but you don't need to rename it. Since the expressions are equivalent, it's still able to be called P.

The formula $\displaystyle x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$ gives you the x-intercepts.

Because a quadratic graph is symmetric, you can find the x co-ordinate of the vertex by averaging the x-intercepts.

The average of the x-intercepts is $\displaystyle -\frac{b}{2a}$. So this is the formula you use.