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Math Help - which quadratic formula do I use

  1. #1
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    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
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  2. #2
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    Quote Originally Posted by bobchiba View Post
    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 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 -\frac{b}{2a}. So this is the formula you use.
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