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!

Rewriting Ln function and getting a wrong function?

  1. Sep 11, 2014 #1
    This is really strange: If i rewrite a potens function i get a function which should not be possible (it does not give the same values). What did i do wrong?

    Y = b*X^a
    lnY = ln(b*X^a)
    lny = lnb + ln X^a
    lnY = lnb + a*lnx

    we raise both sides to e

    Y = b+ e^(a*lnx)=b+( e^lnx)^a = b + X^a

    I think b+ x^a is very diffrent that b*x^a ????
     
  2. jcsd
  3. Sep 11, 2014 #2

    ShayanJ

    User Avatar
    Gold Member

    The point is, [itex] e^{x+y}=e^x e^y [/itex] but you assumed [itex] e^{x+y}=e^x+e^y [/itex] which is wrong!
     
  4. Sep 11, 2014 #3
    So are you saying that when i raise the right side of an equation (lnb + a*lnx ) to e, I should raise it like this e^(lnb + a*lnx ) and not the individual terms?
     
  5. Sep 11, 2014 #4

    ShayanJ

    User Avatar
    Gold Member

  6. Sep 11, 2014 #5

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    That's exactly what you used when you went from ln(ab) to ln(a)+ ln(b).
     
  7. Sep 11, 2014 #6

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    You made the same basic error in another thread, and in that thread several people explained to you where your mistake lies. Please pay attention to what people are explaining to you!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted