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Question On De Moivre IN Complex no.s-Honours maths

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  • 05-06-2008 8:28pm
    #1
    mike-gunners
    Closed Accounts Posts: 33 ✭


    Can any one solve this for me:

    Q:Using De moivre's Theorm, write sin5(-)/sin(-) in terms of cos(-)
    *(-) represents theta


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    #2
    BMH
    Closed Accounts Posts: 804 ✭✭✭BMH


    You sure you need to use de Moivre? You can multiply top and bottom by sinθ and use formulae to change to cosθ's


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    #3
    Cokehead Mother
    Closed Accounts Posts: 773 ✭✭✭Cokehead Mother


    cos5A + isin5A = (cosA + isinA)^5

    Expand the right hand side. Let imaginary = imaginary and you'll be able to get sin5A in terms of sinA, which you can obviously divide by sinA quite easily. Then you can change write it in terms of cosA using (cosA)^2 + (sinA)^2 = 1.
    BMH wrote: »
    You sure you need to use de Moivre? You can multiply top and bottom by sinθ and use formulae to change to cosθ's

    This could come up in Q3 and if it does they will probably tell you to use DeMoivre's and give your 0 if you do it with trig identities, regardless of whether it's right or not.


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