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Calculus: Derivatives Problem #2

  1. Jan 24, 2008 #1
    [SOLVED] Calculus: Derivatives Problem #2

    1. The problem statement, all variables and given/known data
    The graph of y=2x^3+24x-18 is


    2. Relevant equations
    y '=6x^2+24
    y ''=12x

    3. The attempt at a solution
    The answer to this is "increasing for all values of x"
    But I want to know why...if substituting -1 for x will make the problem a negative.
     
  2. jcsd
  3. Jan 24, 2008 #2

    Dick

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    What's the question? If you want to find where y is increasing it's where is y' positive. You don't think y' is positive for all x?
     
  4. Jan 24, 2008 #3
    because when you plug in -1 for y=2x^3+24x-18 the end result is a negative not positive.

    The other choices to the questions are:
    b.decreasing for all values of x
    c.only increasing for values of x on the interval (-infinity,-2)U(2,+infinity)
    d.only increasing for values of x on the interval (-2,2)
    e. only decrasing for values of x on the interval (-infinity,-2)
     
  5. Jan 24, 2008 #4
    NVM, TOTAL IDIOT HERE... I know what you're saying now. Thanks.
     
  6. Jan 24, 2008 #5

    Dick

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    The question isn't where y is negative, it's where y in increasing. y can be increasing even if it's negative.
     
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