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Math Help - Trig identity

  1. #1
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    Trig identity

    Hi, got an identity I need to prove.

    [tex]\frac{(cos 3A - cos 7A)} / {(sin 7A+sin 3A)}=tan 2A[tex]

    Any help would be appreciated.

    Thanks in advance.
    Last edited by jo74; June 8th 2009 at 09:43 AM.
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  2. #2
    MHF Contributor alexmahone's Avatar
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    Last edited by alexmahone; June 8th 2009 at 08:42 PM.
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  3. #3
    MHF Contributor Amer's Avatar
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    Quote Originally Posted by jo74 View Post
    Hi, got an identity I need to prove.

    [tex]\frac{(cos 3A - cos 7A)}{(sin 7A+sin 3A)}=tan 2A[tex]

    Any help would be appreciated.

    Thanks in advance.
    \frac{(cos 3A - cos 7A)}{(sin 7A+sin 3A)}=tan 2A

    try to solve it with these

    sinx + siny = 2 sin(\frac{x+y}{2}) cos(\frac{x-y}{2})

    cosx - cosy = -2 sin(\frac{x+y}{2}) sin(\frac{x-y}{2})

    note sin(-x) = - sin(x)
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