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Laplace Transform Help

  1. Feb 10, 2013 #1
    1. The problem statement, all variables and given/known data
    Find the laplace transform of:

    v(t) = sin(7t)u(t-3)

    2. Relevant equations

    u(t) is step response


    3. The attempt at a solution

    I know that taken alone the laplace of:

    sin(7t) is 7/(s^2+49)

    u(t-3) is (1/s)*(e^-3s)


    I don't understand how I would figure out the answer to this problem if I have u(t-3). For some reason, seeing the u(t-3) rather than u(t) is really making things hard to understand since I don't know how u(t-3) would be integrated.


    I considered setting t-3 =x which would yield:

    v(t) =sin(7x+21)u(x)

    v(t) = sin(7x)cos(21) + cos(7x)sin(21)

    Laplace = [7*cos(21) + s*sin(21)]/[s^2 + 49]

    but I don't know if I'm that's the right answer.
     
    Last edited: Feb 10, 2013
  2. jcsd
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