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Math Help - by inspection

  1. #1
    Member pberardi's Avatar
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    by inspection

    Solve: (2t)ds + s(2+s^2t)dt = 0
    tds + sdt + (1/2)s^(2)tdt
    by inspection int factor is 1/st
    [d(st)]/st + (1/2)sdt
    integrate
    ln|st| + 4s^2

    Is this right at all?
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  2. #2
    ixo
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    Quote Originally Posted by pberardi View Post
    Solve: (2t)ds + s(2+s^2t)dt = 0
    tds + sdt + (1/2)s^(2)tdt
    by inspection int factor is 1/st
    [d(st)]/st + (1/2)sdt
    integrate
    ln|st| + 4s^2

    Is this right at all?
    is the problem: Solve: (2t)ds + s(2+s^2t)dt = 0 then all the rest is your input?
    What exactly is the question?
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  3. #3
    Member pberardi's Avatar
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    Yes sorry the problem is solve: (2t)ds + s(2+s^2t)dt = 0
    The following is my attempt at the solution. I was wondering
    if my attempt is correct and if it isn't how to do it correctly.
    Thanks.
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  4. #4
    ixo
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    (2t)ds + s(2+s^2t)dt = 0 () just mean multiply and if your multiplying you can rearrange everything your multiplying.

    2dts + dts(2+s^2t) = 0 then you can subtract from one side and add it to the other

    dts(2+s^2t) = 0 - 2dts

    dts(2+s^2t) = - 2dts then divide to cancel

    \frac {dts(2+s^2t)}{dts} = \frac{- 2dts}{dts}

    2+s^2t = - 2 then subtract again

    s^2t = - 2 - 2

    s^2t = - 4 then divide again

    s^2 = \frac {- 4}{t} or t = \frac {- 4}{s^2} depending on what your solving for.


    Not trying to be mean but for future note it would be best to post the question just as its asked. Then clearly under it say ... this is what i think: and show your attempt to solve hope this helps
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  5. #5
    Member pberardi's Avatar
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    Thanks for the help and you are right I should have said something like "this is my attempt".

    One other thing. I am not familiar with this method of solving diff equations. This problem is supposed to be solved by the use of an integrating factor. Since it is not exact and since the form (Mpartial t - Npartial s)/N and (Mpartial t - Npartial s)/-M do not give functions of either t or s alone then I must use inspection.

    I am sorry if I am not making sense because it is only my third week in this class, but is my method of inspection correct? Could someone possibly please help me with the correct method of inspection?

    Thanks.
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  6. #6
    ixo
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    hmm ok let me take another look at it real quick.. and try it by inspection.

    (2t)ds + s(2+s^2t)dt = 0

    (2t)ds + 2sdt+ds^3t^2 = 0

    s^3t^2d + 2std + s(2t)d= 0

    Sorry i'm not sure on inspection factoring..
    Last edited by ixo; February 10th 2009 at 10:17 PM.
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  7. #7
    Member pberardi's Avatar
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    inspection still not resolved

    Thanks for your help ixo. Is there anyone else that is willing to help me out with this?
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