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Homework Help: Complex Numbers and amplitude

  1. Mar 9, 2010 #1
    Hi this isn't homework, just a practice problem I already have the answer too for my waves class:
    z=sin(wt)+cos(wt)
    Express this in the from Z=Re[Aej(wt+[tex]\alpha[/tex])]

    I know how to express sine in the form of cosine, and cosine in the from of a complex exponential, but I don't know how to do this...I need to find the amplitude and [tex]\alpha[/tex]. Can someone help?
     
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  3. Mar 9, 2010 #2

    vela

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    Expand [itex]Re[Ae^{j(\omega t+\alpha)}][/itex] in terms of sin(ωt) and cos(ωt) and compare it to z.
     
  4. Mar 9, 2010 #3
    Well for [itex]Re[Ae^{j(\omega t+\alpha)}][/itex]...the inside equals[itex]Ae^{j(\omega t)}e^{j\alpha}[/tex] and [itex]e^{j\omega t}[/itex]=cos(wt)...I'm not really sure where to go from there.
     
  5. Mar 9, 2010 #4

    vela

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    You need to get the inside into the form x+iy so you can just pick off the x when you take the real part. Don't break the exponential up. Just use Euler's formula on it.
     
  6. Mar 9, 2010 #5
    Ok so cos(wt)+sin(wt)=Re[Acos(wt+a)]+Re(Ajsin(wt+a)....and now..I don't really get what the Re does, I know that means real, but what is the significance of it here? Am I suppose to take out the j or something?
     
  7. Mar 9, 2010 #6

    vela

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    You throw away the imaginary part: Re[x+iy]=x.
     
  8. Mar 9, 2010 #7
    Oh I got it thanks!
     
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