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Homework Help: Trigonometry expansion

  1. Sep 25, 2013 #1
    1. The problem statement, all variables and given/known data

    We know that sin(2⋅x)=2⋅sin(x)⋅cos(x)and cos(2x)=cos^2(x)−sin^2(x). Use the appropriate formula to expand cos(8x) in terms of one of these two formulas.


    2. Relevant equations
    sin(2⋅x)=2⋅sin(x)⋅cos(x)
    cos(2x)=cos^2(x)−sin^2(x)

    3. The attempt at a solution
    I am not looking for help with the solution, I just want to know what the question is asking me to do. Is it asking me to sub cos(8x) into these equations like cos(8x) = cos^2(4x) - sin^2(4x)? If not what is it asking me to do?

    Thank You!
     
  2. jcsd
  3. Sep 25, 2013 #2

    Mark44

    Staff: Mentor

    Yes. Now do the same sort of thing on the cos(4x) and sin(4x) terms.

    Note that the notation cos2(4x) means [cos(4x)]2, and similar for sin2(4x).
     
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