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!

Proof about logarithms

  1. Sep 26, 2015 #1
    1. The problem statement, all variables and given/known data
    Proof xlogby=ylogbx

    2. Relevant equations
    logb is a one to one function, that is, if logbx=logby
    hint take the log in both sides of the equation and use the previous hint

    3. The attempt at a solution
    xlogby=ylogbx
    logby log x = logbx logy
    logby / logbx=logy/log x
    logby/x=log(y/x)

    the book says that log(r) without any base indicates that the statement is true for any base so we can say that log(y/x)=logby/x

    so
    logby/x=logby/x

    if this is true then by the hint, x=y, so the original statement must be true
     
  2. jcsd
  3. Sep 26, 2015 #2

    andrewkirk

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    What does that mean? 'log(r)' is not a statement - it contains no verb. When log is written without a base the usual convention is for it to mean log base e, although in older texts it is sometimes taken to mean log base 10.

    The proof is short. Just apply the fact that ##a^c\equiv e^{c\log a}## to both sides and use the fact that ##\log_d f=\frac{\log f}{\log d}##. Here I use the standard convention of using ##\log## for ##\log_e##
     
  4. Sep 26, 2015 #3
    look this is exactly what the book says , "When log(x) is used without any base being mentioned, it means that the statement is true for any base", I think that mine would make sense if apply log base b in both sides of the equation and then I go backwards.
     
  5. Sep 26, 2015 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    That is a trivial statement because both sides are the same. You reduced the equation you have to show to something trivially true: done.
    It is much easier to read the proof if you directly take the logarithm to base b.

    Do you really mean x=y? That doesn't have to be true.
     
  6. Sep 26, 2015 #5

    andrewkirk

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That may work alright for some high-level statements, such as 'log z(u) is a linear function of u', but it's hard to think of any equations where it would work. It certainly doesn't work in this instance.

    Ah, now I've worked out what the book means. The author is saying that she will only use the word 'log' without specifying a base when the statement she is making is true in all bases. That is guidance as to what she may do, not as to what you may do. When writing equations, the base needs to either be stated, or unambiguously implied (as per the e convention). The author's statement is a guide to interpreting her text, not a technique you may use in writing proofs.
     
  7. Sep 26, 2015 #6

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    A way to fix that: take the logarithm to base a, where a is an arbitrary positive real number.
    On the other hand, and as I said before, if we want to choose a=b later, we can use log_b the whole time.
     
  8. Sep 26, 2015 #7
    so mine is right
    thank you so much, yes , I applied log base b at the beginning and came up with the trivial statement.
     
  9. Sep 28, 2015 #8
    anyways, it is wrong. I made a stupid mistake of log properties xd.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Proof about logarithms
  1. Logarithm Proofs (Replies: 13)

  2. Proofs (Logarithm) (Replies: 4)

  3. Logarithm Proof (Replies: 2)

Loading...