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Homework Help: Function negative for all values of x

  1. Jun 14, 2008 #1
    1. The problem statement, all variables and given/known data
    Find the range of values of m for which the function 3x^2 -12x + m is negative for all values of x.

    2. Relevant equations
    Is it possible for the function to be negative ? If so, how ?

    3. The attempt at a solution
    I tried using b^2 - 4ac < 0 and the result is m > 12 . However when i tried substituting *13* as the value of m and *1* as the value of x, the result is a positive number.
     
  2. jcsd
  3. Jun 14, 2008 #2

    Defennder

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    I don't think that any such value of m exists. Just picture this intuitively. What does the graph look like? Is it shaped like a smiley expression or sad expression? Moreover, even if the discriminant is less than zero it just means that the graph doesn't cut the x-axis. It doesn't mean that it is below the x-axis.
     
  4. Jun 15, 2008 #3

    Gib Z

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    Defennder is correct: No such value of m exists. Complete the square, or just see that 3x^2 - 12x itself can be made as large as required to cancel out any value of m.
     
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