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Proving Trigonometric Identities

  1. Apr 26, 2009 #1
    1. The problem statement, all variables and given/known data

    cosx - cosy=-2 sin(x + y/2) sin(x - y/2)

    2. Relevant equations
    dont know what identities to use

    3. The attempt at a solution

    ok so when i figure it out, the RHS always comes out to either...

    (cos x sin y - sin x cos y)/2

    or just

    cos x sin y - sin x cos y without the divided by 2 part

    it never equals the LHS=cosx - cosy
  2. jcsd
  3. Apr 26, 2009 #2
    I asked in your previous thread https://www.physicsforums.com/showthread.php?t=309555" if you mean (x+y)/2 or x+(y/2), but you didn't answer. I also think that the rest of us here would like to see the steps you took to get to
    (cos x sin y - sin x cos y)/2 or cos x sin y - sin x cos y.

    Last edited by a moderator: Apr 24, 2017
  4. Apr 26, 2009 #3
    i mean ((x+y)/2)
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