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!

Confusion on this logarithmic equation NEED HELP

  1. Nov 30, 2012 #1
    e^(3+2logx) = (3x-2)*e^3

    I can get down to this point: e^(logx^2)=3x-2 I have checked the solution in the book, and the steps. Turns out it simplifies to x^2 -3x +2=0. The problem is I can't figure out how I pass from e^(logx^2) to x^2, because as far as I know logx isn't the same as lnx.

    Could you please explain me that step?
     
  2. jcsd
  3. Nov 30, 2012 #2

    LCKurtz

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Apparently log x does mean ln x in the context of that problem.
     
  4. Nov 30, 2012 #3
    Yeah but why?
     
  5. Nov 30, 2012 #4

    LCKurtz

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Some texts use the convention that log(x) means natural log if no other base is specified.
     
  6. Nov 30, 2012 #5

    symbolipoint

    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    I was able to find the same quadratic equation as you after taking natural logarithms of both sides, [STRIKE]but I still could not show that the original left side and right side are equal[/STRIKE] (I was thinking the question wronly. SOLVING the equation was intended, NOT proving left and right were equal).

    One way was to take logs of both sides. Another way was to first divide both sides by e^3.
    If divide L and R by e^3,
    e^(2lnx) = 3x-2
    e^(lnx^2)=3x-2
    x^2=3x-2
    Simple and easy to find x.
     
    Last edited: Nov 30, 2012
  7. Nov 30, 2012 #6

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    In "pre-Calulus" text books, and many older "Calculus and higher" text, where using logarithms to do calculations was predominant, "log x" typically meant "common logarithm, base 10" while "ln x" was the "natural logarithm, base e".

    However, in modern, Calculus and above", texts, common logarithms are just never mentioned and "log x" is used to mean "natural logarithm".
     
  8. Nov 30, 2012 #7

    symbolipoint

    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    Post #6:
    Numbers could be written in "scientific notation", using the base-ten system, and the numbers and tables of logarithms could be used in performing computations for the numbers. This is why base-ten logarithms were important and very often used. Our books taught about both log base ten AND log base e. Best of memory is like HallOfIvy said, log was for base ten, and ln was for base of Euler number.
     
  9. Dec 1, 2012 #8

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    However, my point was that most advanced texts simply ignore
    "log base 10" and use "log x" to mean natural logarithm.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Confusion on this logarithmic equation NEED HELP
  1. Logarithm need help (Replies: 6)

Loading...