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Math Help - solve for a

  1. #1
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    solve for a

    Prove that cos a + cos 2a + cos 3a + cos 4a = 4 cos (a/2)cos a cos (5a/2)

    i can prove this .

    hence , find all the values of a between 0 and 180 such that

    cos a + cos 2a + cos 3a + cos 4a =0

    so i know i should start like this

    4 cos (a/2)cos a cos (5a/2)=0

    but i am unsure how to continue here .
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  2. #2
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    Hi thereddevils

    The next step is :

    cos(a/2) = 0 or cos (a) = 0 or cos (5a/2)=0

    Solve each case using general solution of trigonometry, then find the particular solution for a
    Last edited by songoku; August 16th 2009 at 06:51 AM.
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  3. #3
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    Quote Originally Posted by songoku View Post
    Hi thereddevils

    The next step is :

    cos(a/2) = 0 or cos (a) = 0 or cos (5a/2)=0

    Solve each case using general solution of triginometry, then find the particular solution for a

    thanks again songoku .. u r really helpful .
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  4. #4
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    You're welcome again thereddevils

    Glad to help
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