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Differential equation x''[t]+2x[t]+5[t]=sin(t)

  1. Oct 23, 2011 #1
    Solving a differential equation, x''[t]+2x[t]+5[t]=sin(t).
    assumed that f=C Exp[it], and substituting into the equation, i found C=(4-2i)/20, but i don't understand why the solution says that it will become 1/5 Sint -1/10 Cost instead of 1/5 Cost-1/10 Sint.
     
  2. jcsd
  3. Oct 23, 2011 #2
    so we have x''+2x'+5x=sin(t) , you didn't write it like this but i figured this is what you meant and this is how i got the answer.
    then we say x=Ae^(it) and then x'=iAe^(it) then x''=-Ae^(it) , then we plug these in and solve for A. then multiply and divide by the complex conjugate. and then we take the imaginary part because we started with sin(t) , which is the imaginary part of Eulers formula.
    im guessing you took the real part.
     
  4. Oct 23, 2011 #3
    Sorry, i don't get it. When we are plugging it the equations, we write -Ae^(it)+2iAe^(it)+5Ae^(it)=e^(it), right? coz sin is the imaginary part of e^(it). And we solved that A=1/(4+2i), multiplying by its complex conjugate gives (4-2i)/20. and then i don't know how it gets to 1/5Sin[t]- 1/10 Cos[t].
     
  5. Oct 23, 2011 #4

    lurflurf

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    I cannot say where you went wrong as you did not show steps. You should have got
    C=(-1+2i)/20
    C*=(-1-2i)/20
    where * is complex conjugation then a particular solution is

    (C+C*)cos(t)+(C-C*)sin(t)

    note that in general (and here)

    Re(C) cos(t)+Im(C) sin(t)

    is not a particular solution

    Edited to add: I think you used another convention for C, using your convention for C
    Im(C) cos(t)+Re(C) sin(t)
    is a particular solution, you probably just switched them
     
    Last edited: Oct 23, 2011
  6. Oct 23, 2011 #5

    lurflurf

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    Right so we know

    (D2+2D+5)Cei t=ei t
    and conclude

    sin(t)=Im[ei t]
    =Im[(D2+2D+5)Cei t]
    =(D2+2D+5)Im[Cei t]
    =(D2+2D+5)(Im[C]cos(t)+Re(C)sin(t))
     
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