1. Limited time only! Sign up for a free 30min personal 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 questions

Tags:
  1. Oct 11, 2014 #1
    1. The problem statement, all variables and given/known data
    Solve for X

    logx-log(x+11) = -1

    and

    log4x-log4(x+15) = -1

    2. Relevant equations


    3. The attempt at a solution
    log x - log (x+11) = -1
    log (x/x+11) = -1

    I don't know how to solve for X after this point

    log4x-log4(x+15) = -1
    log4 x/(x+15) = -1


    I don't know how to get the X out of the log to solve for X
     
    Last edited: Oct 11, 2014
  2. jcsd
  3. Oct 11, 2014 #2

    Mark44

    Staff: Mentor

    The relationship you need for both problems is this one:
    loga(x) = y is equivalent to x = ay.

    For your first problem, log means log10 (lob base 10).
     
  4. Oct 11, 2014 #3

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    What "base" of logs is used in the first question?

    Anyway, you certainly cannot have what you wrote, which was
    [tex] \log \left( \frac{x}{x} + 11 \right) = -1[/tex]
    which gives ##\log(12) = -1##. Did you mean
    [tex] \log\left( \frac{x}{x+11} \right) = -1?[/tex]
    If so, use parentheses, like this: log(x/(x+11)) = -1. At his point it matters what base you are using for log.

    For a given base ##b##, what number, ##y##, has ##\log_b(y) = -1##? Think about what that actually means.

    BTW: either use X or x, but not both in the same problem.
     
  5. Oct 11, 2014 #4
    I figured them out, thank you.

    log x - log (x+11) = -1
    log (x / (x+11)) = -1
    x/(x+11) = 10-1
    x/(x+11) = 0.1
    x=0.1(x+11)
    x=0.1x+1.1
    x-0.1x=1.1
    0.9x=1.1
    x=1.222...

    log4x-log4(x+15) =-1
    log4(x/(x+15) = -1
    x/(x+15)= 4-1
    x/(x+15) = 0.25
    x=0.25(x+15)
    x=0.25x+3.75
    x-0.25x=3.75
    0.75x=3.75
    x=5
     
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: Log questions
  1. Log question (Replies: 4)

  2. Log Question ! (Replies: 6)

  3. Log question (Replies: 1)

  4. Log question (Replies: 11)

Loading...