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!

Derivatives of Log

  1. Nov 30, 2008 #1
    1. The problem statement, all variables and given/known data
    Question is
    f(x) = log10(x/x-1), find f `(x)


    2. Relevant equations
    so I used the formula
    d/dx loga(u) = 1/uLNa . (d/dx U)


    3. The attempt at a solution
    (1/(x/x-1)*ln10)*1/(x-1)^2 I used quotent rule for d/dx U part.

    editing

    Final Answer I got is

    1/x(x-1)ln10
    or
    1/x2-x ln10
    Please help
    thank you
     
    Last edited: Nov 30, 2008
  2. jcsd
  3. Nov 30, 2008 #2

    lurflurf

    User Avatar
    Homework Helper

    it may be easier to use the log property
    log(ab)=log(a)+log(b)
    log(x/(x-1))=log(x)-log(x-1)
    [log10(x/(x-1))]'=[log(x/(x-1))]'/log(10)
    =[log(x)-log(x-1)]'/log(10)
    =([log(x)]'-[log(x-1)]')/log(10)
     
  4. Nov 30, 2008 #3
    so many answer is not right
     
  5. Nov 30, 2008 #4

    lurflurf

    User Avatar
    Homework Helper

    Just a sign error
    Quotient rule is
    (u/v)'=(u'v-uv')/v^2
    so
    (x/(x-1))'=(x'(x-1)-x(x-1)')/(x-1)^2
    =(1(x-1)-x(1))/(x-1)^2
    =-1/(x-1)^2
    not
    1/(x-1)
    also watch your grouping symbols
    1/x2-x ln10
    is confusing
    try
    1/[(x^2-x)ln 10]
    or

    [1/(x^2-x)]/ln 10
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Derivatives of Log
  1. Derivative of log (Replies: 18)

Loading...