1. Not finding help here? Sign up for a free 30min 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!

Log substitution problem

  1. Apr 25, 2007 #1
    1. The problem statement, all variables and given/known data

    (Idk how to put in the equation to make sense, therefore it is at the link below)

    2. Relevant equations



    3. The attempt at a solution

    Here is all I have done. Something just isn't right...there should be 3 answers (in the back of the book) because there is a cube involved...

    http://i123.photobucket.com/albums/o318/trashfile_bucket/Trash/2007-04-25-2009-26_edited.jpg
     
  2. jcsd
  3. Apr 25, 2007 #2

    mjsd

    User Avatar
    Homework Helper

    note:
    [tex](\log x)^3 \neq \log (x^3)[/tex] ie. 2nd step is wrong

    hint: a make substitution: y = log x and solve for y first then x.
     
  4. Apr 25, 2007 #3
    The first thing you've done is to cube both sides. That's ok but it should give you
    [tex] ( log(x) )^3 = log(x) [/tex]
    Since the whole log(x) is cubed, you can't move the 3 down (that's only if the x was cubed).

    But what you can do is take all the terms over to one side and then you just have to solve a cubic (which will give you 3 solutions). You may want to make it easier to see by introducing a new variable, [tex] u = log(x) [/tex] for example.
     
  5. Apr 25, 2007 #4
    Good to go!
     
  6. Apr 27, 2007 #5
    and the solutiions should probbably be, couse i just glanced at it,:
    x_1=1
    x_2=y,(if the base of the logarithm is y, couse i could not see it clear)
    x_3=1/y
     
    Last edited: Apr 27, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Log substitution problem
  1. Substitution problem (Replies: 5)

  2. Log problem (Replies: 5)

  3. Log problem (Replies: 1)

  4. Log and > problem (Replies: 2)

  5. Log problem (Replies: 6)

Loading...