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Laplace transformation

  1. May 15, 2014 #1
    1. The problem statement, all variables and given/known data
    Find Laplace transformation for functions ##f(t)##:
    a) ##5cos(7t+\pi /4)##
    b) ##e^{3t}sintcost##


    2. Relevant equations



    3. The attempt at a solution

    a) I know that for ##cos(\omega t)## the laplace is ##\frac{s}{s^2+\omega ^2}## but what can I do with that ##\pi /4## ?

    I believe I would have to use this identity ##f(t)^{'}=sF(s)+f(0)## but I don't know how o use it...

    Could somebody please show me that?
     
  2. jcsd
  3. May 15, 2014 #2

    LCKurtz

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    Use the addition formula for ##\cos(a+b)## before taking the transform.
     
  4. May 15, 2014 #3
    OMG, How can I be so stupid! -.-

    Thanks LCKurtz!
     
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