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Homework Help: Another complex r00ts 2nd ODE! gahh! i <3 DiffEQ

  1. Feb 17, 2006 #1
    Look whos back! I ran into another problem, i redid the problem twice and i keep coming out with the same answer!
    here is what i have:
    http://img139.imageshack.us/img139/6205/lastscan5qw.jpg [Broken]
    This is waht I submitted which was wrong:
    http://cwcsrv11.cwc.psu.edu/webwork2_files/tmp/equations/d3/290d3b430db500dfaef04358236c001.png [Broken]
    This is what
    Thanks!
     
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Feb 17, 2006 #2
    If you're having trouble doing it with sine and cosine, do it the old fashioned Ae^r1x + Be^r2x way using the complex roots. (It's an alternative method and chances are you'll make less mistakes)
     
  4. Feb 17, 2006 #3

    HallsofIvy

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    If that is what you actually submitted, you might want to compare it closely to your handwritten answer- do you notice some missing parentheses?
     
  5. Feb 17, 2006 #4
    Pseudo, this program that evaluates the expressions tells us to not use that form becuase it won't evaluate complex expressions inside the e^, so I have to use sin/cos. :(

    Thanks i didn't catch that halls but it still told me, do not pass go, do not collect 200 dollars. I think i shall e-mail the professor and see if its corrrect and the program might just b f'ed up.
    http://cwcsrv11.cwc.psu.edu/webwork2_files/tmp/equations/3d/4a14ab68c9143ae5c68ed8af6e5ba91.png [Broken]
     
    Last edited by a moderator: May 2, 2017
  6. Feb 17, 2006 #5
    Again, like last time, your differentiation is wrong! You seem to forget that there is a fraction in the exponential; your didn't include the denominator (1000) in the differentiation! It is the exact same mistake you did last time. I suggest you carefully go through the differentiation again.
     
    Last edited: Feb 17, 2006
  7. Feb 17, 2006 #6
    Okay i redid it, i'm not sure if its right t hough becuase the webhomeworks happen to be down. but this is what I got:
    http://img212.imageshack.us/img212/6735/lastscan1rm.jpg [Broken]
     
    Last edited by a moderator: May 2, 2017
  8. Feb 17, 2006 #7
    I ended up with the equation

    [tex]7=\frac{14}{1000}-\frac{\sqrt{2951}}{1000}B[/tex]

    so B should be

    [tex]B=-\frac{6896}{\sqrt{2951}}[/tex]

    However, I could have made a mistake. :smile:
     
  9. Feb 18, 2006 #8
    Hm...I tried my answer and your answer, switching the signs as well but can't seem to get it right.
    http://cwcsrv11.cwc.psu.edu/webwork2_files/tmp/equations/02/a1ed9e2620a00e5df238eb7b1954831.png [Broken]
     
    Last edited by a moderator: May 2, 2017
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