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Homework Help: Negative logarithm within an equation problem?

  1. Jun 2, 2015 #1
    Hello,

    I know that a negative logarithm is undefined.

    But I am faced with an equation like this:
    -1 x (-2)^(n-1) = -16777216
    I divided it by -1 to give (-2)^(n-1) = 16777216
    And then took logs to get (n-1) log(-2) = log 16777216

    Since I can't work out the log of a negative number, what do I do?

    I know that the answer is 25 if i use log (2) but I don't get that :/
     
  2. jcsd
  3. Jun 2, 2015 #2

    Mark44

    Staff: Mentor

    Logarithms can be negative, as for example log(.1) = -1. What you probably meant was that you can't take the log of a negative number, assuming that we're dealing with the real-valued log function.
    You could make an assumption about n. In this equation, (-2)^(n-1) = 16777216, if n is an odd integer (so that n - 1 is even), then (-2)n - 1 will be equal to (2)^(n-1) = 16777216. I would look at two cases: one where n is odd, and the other where n is even.
     
  4. Jun 2, 2015 #3
    Thank you for quick response.

    So it works similar to the case where (-1)^even = positive and (-2)^odd = negative...
    so if n-1 was odd (n was even), then the equation would be unsolvable...

    It's an easier solution than I thought... It's nice to know why I turned that expression positive for all these years and got the right answer!

    Edit: Yes, I did mean taking a log of a negative number.
     
    Last edited: Jun 2, 2015
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