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Homework Help: Why is this?

  1. Apr 23, 2012 #1
    1. The problem statement, all variables and given/known data

    http://www.wolframalpha.com/input/?i=((1+/+x)+++2)+/+(3(2x+++lnx))^(2/3)+=+0



    Solve the equation f'(x) = 0

    f(x) = (ln x + 2x)^(1/3)


    3. The attempt at a solution

    As you can see, for the function

    ((1 / x) + 2) / (3(2x + lnx))^(2/3) = 0

    I can get a value for x which is - 1 / 2

    But my text book says "no solution"

    Why is that?
     
  2. jcsd
  3. Apr 23, 2012 #2

    Mark44

    Staff: Mentor

    x must be positive in order for ln(x) to be defined.
     
  4. Apr 23, 2012 #3
    Oh wow, I totally missed that. Thank you very much. :)
     
  5. Apr 23, 2012 #4
    That does not mean that wolfram alpha is wrong however. Wolfram alpha uses a definition of the logarithm which is also defined for negative numbers. This definition uses complex numbers.

    However, you likely did not encounter complex numbers and complex logarithms yet, so in your problem x=-1/2 is not a valid solution as the logarithm is not well-defined.

    This is a bit the danger of using computer software. Their solutions are not wrong, but they can be different solutions from what you want.
     
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