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!

Help me find y

  1. Jan 7, 2007 #1
    1. The problem statement, all variables and given/known data

    If [tex]ln|y| - ln|y-a| = b[/tex] find y

    2. The attempt at a solution

    [tex]ln\frac{y}{y-a} = b[/tex]

    [tex]\frac{y}{y-a} = e^b[/tex]

    [tex]y-e^by = -ae^b[/tex]

    [tex]y (-e^b) = -ae^b[/tex]

    [tex]-y = \frac{-ae^b}{e^b}[/tex]

    [tex]y = a[/tex]

    I know thats not right.. let me know where I went wrong
     
  2. jcsd
  3. Jan 7, 2007 #2

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    Here's your mistake: this line should read [tex]y(1-e^b)=-ae^b [/tex]
     
  4. Jan 7, 2007 #3
    thanks.. so...

    [tex]y-e^by = -ae^b[/tex]

    [tex]y = \frac{-ae^b}{y-e^b}[/tex]

    [tex]y^2 = \frac{-ae^b}{-e^b}[/tex]

    [tex]y^2 = a[/tex]

    [tex]y = \sqrt a[/tex]
     
  5. Jan 7, 2007 #4
    This is wrong you divided both sides by (y-eb) but you didn't divide correctly on the left hand side, it should be 1.

    [tex]1 = \frac{-ae^b}{y-e^b}[/tex]
     
  6. Jan 8, 2007 #5
    snowJT what are you doing? why did you divid by [tex]y-e^b[/tex] ?

    continue from what cristo said then divid the right hand side by [tex]1-e^b[/tex]

    then multiply top and bottom of the fraction on the right hand side by minus

    and do ln of both sides and use the rules of log to get y

    (if my advice is wrong or there is a simpler method some one please tell me because i actually got two answers and posted the one that might be more correct)
     
  7. Jan 8, 2007 #6

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    This is correct.
    I don't see why you'd take the log. We are seeking y, and after performing the operations listed above, we have y on the left hand side!
     
  8. Jan 8, 2007 #7
    oh, i thought (to get y in a simpler form) you could expand out the ln then do e again, but that wouldn't work and i should know that! (i think i'm getting tired!)

    i don't think i should give out any advice next time unless i'm awake and certain, i just feel that i owe people lots of help because i got so much help from this site lol
     
  9. Jan 9, 2007 #8
    oh, lol.. no I don't know what I was thinking..
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Help me find y
  1. Find y (Replies: 2)

Loading...