1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    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


    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).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook