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Homework Help: Trig identity

  1. Apr 4, 2010 #1
    1. The problem statement, all variables and given/known data
    I have to prove that 3A=(cos(2A)-cos(4A))/(sin(4A)-sin(2A))

    2. Relevant equations
    All of the trig identites.

    3. The attempt at a solution
    Well, I know I can expand tan(3A) to sin(3A)/cos(3A). After this, I have no idea what to do. I do not know any way to simplify or expand the right side of the equation. If someone can help me out by just showing the me the first step, if possible, then I can try to solve the rest of the identity by myself.
  2. jcsd
  3. Apr 4, 2010 #2


    Staff: Mentor

    I don't think you have written the problem correctly. I believe you meant to write
    tan(3A) = (cos(2A)-cos(4A))/(sin(4A)-sin(2A))

    If so, start with the right side and notice that 2A = 3A - A, and 4A = 3A + A.
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