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Homework Help: Missing a step:

  1. Apr 15, 2006 #1
    I'm not sure why I can't see what step is occuring here, but I could use some help:

    [tex]G(s) = \frac {30}/{(s^2 + 6s +20)}[/tex]

    [tex]H(s) = \frac {k} /{s+8}[/tex]

    Characterisic equation: [tex]1+ G(s)H(s)[/tex]


    [tex]1 + \frac ({30k} /{(s^2+6s+20)(s+8))} = 0[/tex]

    Then, the step I'm not sure about:

    [tex](s^2+6s+20)(s+8) + 30k = 0[/tex]

    How do I get to that step? For some reason I can't see the simple algebra that is used.
    Last edited: Apr 15, 2006
  2. jcsd
  3. Apr 15, 2006 #2


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    Gold Member

    Is this what you meant to write?

    It looks like they just multiplied both sides by [tex](s^2+6s+20)(s+8)[/tex]
  4. Apr 16, 2006 #3
    Doing a little controls theory are we? :biggrin: Yep, just mutliply both sides by that.
  5. Apr 17, 2006 #4
    Yup, control theory. I don't know why I couldn't see that step, I think it's time to take a break. :rofl:

    Thanks guys.
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