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Help with solving Laplace Transform problem

  1. Feb 5, 2017 #1
    1. The problem statement, all variables and given/known data
    Solve Laplace Transform L{tsin(2t)sin(5t)}

    2. Relevant equations
    cos(bt)=s/s^2+b^2
    trig identity (product identity): sin(a)sin(b)=1/2[cos(a-b)-cos(a+b)
    t^nf(t)=(-1)^nd^n/ds^nF(S)
    (the template is complicated for me to use. Srry for the inconvinience)
    3. The attempt at a solution
    So far I have 1/2L{cos(-3)-1/2L{cos(7)}
    1/2(s/s^2+4)-1/2(s/s^2+49)

    Am I right?
     
  2. jcsd
  3. Feb 5, 2017 #2

    Ray Vickson

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    No, not even close.

    BTW: you wrote
    $$(1)\hspace{1.5cm}1/2 \left(\frac{s}{s^2}+4 \right) - 1/2 \left( \frac{s}{s^2} + 49 \right),$$
    which equals ##(1/2)(-45)##. If you mean
    $$(2) \hspace{1.5cm}1/2 \left(\frac{s}{s^2+4} \right) - 1/2 \left( \frac{s}{s^2 + 49} \right),$$
    then you need to use parentheses, like this: 1/2 s/(s^2+4), etc. (When I said your answer was wrong I assumed you meant (2), not (1).)
     
  4. Feb 5, 2017 #3

    LCKurtz

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    Same comment

    What happened to the ##t## variable in that last line?
     
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