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Math Help - Complex numbers/trigonometry:

  1. #1
    Junior Member
    Joined
    Sep 2009
    Posts
    26

    Complex numbers/trigonometry:

    I have come so far:

    =cis(4∏/5) + cis(6∏/5) + cis(-8∏/5) + cis(8∏/5)
    =2cos(4∏/5) + 2cos(8∏/5)
    =?

    How do I solve this without using a calculator (answer should be -1)?
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  2. #2
    Senior Member
    Joined
    Mar 2009
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    378
    Check out the trig identities.
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