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Thread: Another Log Problem: logY^3 = logx(x+4)^2

  1. #11
    Elite Member
    Join Date
    Sep 2012
    Quote Originally Posted by Ted_Grendy View Post
    Hello all

    I have another log problem which I was hoping someone could help me with.

    I have the following:-

    logY^3 = logx(x+4)^2

    I have been told that the above statement is the same as:-

    Y^3 = x(x+4)

    The bit I don't understand is where does the log go?
    What law allows the log to be removed from both sides?

    Can anyone help?

    Thank you.

    (assume base 10 log)
    This question belongs in intermediate algebra or perhaps even pre-calculus, but it is an excellent question.

    Logarithm functions, regardless of base, are members of a class of functions known as invertible. There is a theorem that proves

    [tex]\text {If } f(x) \text { is invertible, then } f(x) = f(y) \implies x = y.[/tex]

    It can be shown that logs are invertible (in my youth, when logs were actually used for computations, we had to work with the inverse called an antilog).

    But if you do not want to work through the proof that logs are invertible, a proof that is very advanced, you can just say that one of the laws of logarithms is

    [tex]log_a(x) = log_a(y) \implies x = y.[/tex]
    Last edited by JeffM; 12-06-2018 at 09:28 AM.


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