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Math Help - Complex Number--De Moivres Theorm

  1. #1
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    Complex Number--De Moivres Theorm

    Can any body help with this one.

    Evaluate the following using De Moivres Theorm's.
    (2+i)^3
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  3. #3
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    Quote Originally Posted by heatly View Post
    Can any body help with this one.

    Evaluate the following using De Moivres Theorm's.
    (2+i)^3
    Start by converting 2 + i to Polar form...
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  4. #4
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    I did that and got,
    |2+i| = sqrt 5
    z=(sqrt(5))^3 (cos pi/2 + i sin pi/2)
    ......
    Which gives...0+11.18i ???

    The ans is z= 2 + 11i
    Last edited by heatly; July 29th 2010 at 02:51 AM.
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  5. #5
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    I don't think you computed the angle correctly. Try \theta=\tan^{-1}(\text{Im}(z)/\text{Re}(z)).
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  6. #6
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    thankyou very much you are correct Ackbeet.
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  7. #7
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    You're welcome. Have a good one!
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