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!

Problem with a logarithmic rule

  1. Jan 30, 2013 #1

    Please could some one help me with the rules of logarithms to understand why inverting the numbers in the bracket and change the minus sign to a plus sign gives the same result?

    Thank you
  2. jcsd
  3. Jan 30, 2013 #2


    User Avatar
    2017 Award

    Staff: Mentor

    Try b=-1
  4. Jan 30, 2013 #3


    Staff: Mentor

    ln(A/B) = ln(A) - ln(B) = -[ln(B) - ln(A)] = -ln(B/A)
  5. Jan 30, 2013 #4
    for the equation on the right: t= ( ln (N0/N) ) / λ:

    if you multiply top and bottom of the fraction by -1 you get

    t= (-1 ln (N0/N)) / -λ

    You have -1 ln (N0/N) which is equal to ln( (N0/N)^-1 ) by the power rule. The (N0/N)^-1 inside the log flips to (N/N0) from the negative exponent and you get:

    t= ( ln (N/N0) ) / -λ.
  6. Jan 31, 2013 #5
    Fantastic thanks to you all
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook