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Homework Help: Quadratic formula question

  1. Mar 5, 2010 #1
    1. The problem statement, all variables and given/known data
    .002x - .000001x^2 = .50



    2. Relevant equations
    -b+-sq.rt.((b^2)-(4ac))/2a



    3. The attempt at a solution
    Plugging a=-.000001, b=.002, and c=-.5 does not get the the correct answer. x is supposed to be 292.89. I can't remember any other way of going about trying to get this answer. Any suggestions?
     
  2. jcsd
  3. Mar 5, 2010 #2
    The formula is

    ( -b +- sqrt(b^2-4ac))/(2a)
     
  4. Mar 5, 2010 #3

    Mentallic

    User Avatar
    Homework Helper

    As elibj123 said, the quadratic formula has a [itex]\pm[/itex] to consider. There are two solutions to any quadratic.

    So you need to solve [tex]x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}[/tex]

    You'll get your solution of 292.89 if you take [tex]x=\frac{-b-\sqrt{b^2-4ac}}{2a}[/tex]

    but you'll also get a solution of approx 1700 if you solve [tex]x=\frac{-b+\sqrt{b^2-4ac}}{2a}[/tex]

    Both solutions are correct, and you can check this by substituting your values of x back into the original equation .002x - .000001x^2 = .50
    If your values of x are correct, the Left-hand side should approximately equal the right-hand side (depending on the approximation of your values of x).
     
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