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What am I doing wrong?

  1. Jul 6, 2005 #1
    Hello

    I have to prove that

    (cos 3x - Cos 7x) / (Sin 3x + Sin 7x) = Tan 2x

    So this is what I am doing

    -2 Sin (3x + 7x)/2 Sin (3x-7x)/2
    --------------------------------
    2 Sin (3x + 7x)/2 Cos (3x-7x)/2

    -2 Sin 5x Sin -2x
    -----------------
    2 Sin 5x Cos -2x

    Sin 2x
    -------
    -cos 2x

    Which equals -Tan 2x. What am i doing wrong??

    Thanks
     
  2. jcsd
  3. Jul 6, 2005 #2
    Cos(-2x)=Cos(2x)
    Then i believe everything is ok.
     
  4. Jul 6, 2005 #3
    Thanks for your help.
     
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