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Homework Help: Expressing as complex numbers

  1. Mar 14, 2008 #1
    How do I express the following in the form a+ib?

    [tex] (\sqrt{-4} - 1) (\sqrt{-9}) [/tex]

    My attempt:
    (2i - 1)(3i)
    6i-3i = 3i
    Last edited: Mar 14, 2008
  2. jcsd
  3. Mar 14, 2008 #2


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    Close, but (2i)(3i)=6i2=-6, not 6i.
  4. Mar 14, 2008 #3
    Can you tell me why the answer -6-+3i has plus or minus 3i?

    Also can you tell me the answer to (3+i)^2 - (3+i)^2
    The answer I got was 0.
    Last edited: Mar 14, 2008
  5. Mar 14, 2008 #4


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    You have to define what you mean by sqrt(-n) for n>0. You could choose it to be either i*sqrt(n) or -i*sqrt(n). That's your sign ambiguity.
  6. Mar 14, 2008 #5


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    What was that the answer to?? Surely not the (2i-1)(3i) problem you were just talking about!

    The answer to a2- a2 is 0 no matter what a is!
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