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Homework Help: 2 complex roots 2nd ODE, did I mess up finding a constant?

  1. Feb 16, 2006 #1
    It is me again, 2 problems later I ran into another problem, i've submitted it a few different times but still is incorrect. Anyone see my mistake? I entered this as the answer:
    http://cwcsrv11.cwc.psu.edu/webwork2_files/tmp/equations/a0/13092fac04d4a01ec22b57e193ed051.png [Broken]

    Here is the problem:
    Find y as a function of t if
    81y'' + 126y' + 79y = 0,
    y(0) = 2, y'(0) = 9 .
    y =

    Here is my work:
    http://img132.imageshack.us/img132/9167/lastscan5tu.jpg [Broken]

    Any help would be fantastical.
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Feb 16, 2006 #2
    Hmm...I got [tex]B=\frac{95}{\sqrt{30}}[/tex] but I could have made a mistake. I think there is a mistake in your differentiation. Check it again. I can see that you have multiplied in the 7 coming from differentiation of the exponential, but what happened to the 9? Remember that [tex]\frac{d}{dt}e^{-\frac{7}{9}t}=-\frac{7}{9}e^{-\frac{7}{9}t}[/tex].
  4. Feb 16, 2006 #3


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    I find that working through a problem twice is usually a good way to catch arithmetic errors. Unless you are suffering from a systematic misunderstanding, you will usually not make the same mistake twice.
  5. Feb 16, 2006 #4
    Ahh yes, i did it again and got the same answer as you did assyrian, thanks a ton! http://cwcsrv11.cwc.psu.edu/webwork2_files/tmp/equations/bb/6a9d9ea040028c2ece7c021178bc261.png [Broken]
    Last edited by a moderator: May 2, 2017
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