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Laplace transform tcos(4t)

  1. May 6, 2015 #1
    1. The problem statement, all variables and given/known data
    f(t)=tcos(4t)


    2. Relevant equations

    tnf(t)=(-1)n dF(s)/dsn

    3. The attempt at a solution
    I don't understand why this formula is giving me the oppiste sign of the answer.

    If I apply the formula I get

    (16-s2)/(s2+16)2

    Because n=1 I need to multiply by a negative but this yields the incorrect answer.
     
  2. jcsd
  3. May 6, 2015 #2
    Yours looks different than the one we use. Try it as: qdZ4Kvq.jpg
     
  4. May 6, 2015 #3

    LCKurtz

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    $$-\frac{-s^2+16}{(s^2+16)^2}=\frac{s^2-16}{(s^2+16)^2}$$which is correct.
     
  5. May 6, 2015 #4
    His answer doesn't have a negative in front?
    Edit: nevermind, looking at wrong thing. Carry on
     
  6. May 6, 2015 #5
    But that's not what I'm getting.

    Taking the derivative of s/(s2+16)

    = (s(2s)-(s2+16)/(s2+16)2

    now multiply the result with -1 and we get the opposite sign for the answer.
     
  7. May 7, 2015 #6

    LCKurtz

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    That is because your quotient rule is wrong.
     
  8. May 7, 2015 #7
    Ah I see. Thank you!
     
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