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Difficulty proving identity

  1. Oct 16, 2006 #1
    can anyone help[ me prove the following identity i keep on ending up in a dead end

    tan3A=3tanA-tan^3 A/ 1-3tan^2 A
    thank you
  2. jcsd
  3. Oct 16, 2006 #2
    you know that [tex] \tan(A+B) = \frac{\tan A + \tan B}{1-\tan A \tan B} [/tex]. Setting [tex] A = B [/tex] we get [tex] \tan 2A = \frac{2\tan A}{1-\tan A \ tan A} [/tex]. So what is [tex] \tan(A+2A) [/tex]? Thus work from the left to the right.
    Last edited: Oct 16, 2006
  4. Oct 18, 2006 #3
    i end up with
  5. Oct 18, 2006 #4
    what am i doing wrong
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