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Help proving an identity

  1. Jun 13, 2009 #1
    1. The problem statement, all variables and given/known data
    Prove this is an identity:
    sin2(x)-sin2(x)=sin(x+y)sin(x-y)


    2. Relevant equations
    N/A


    3. The attempt at a solution
    I have made a lot of attempts but can not get one side to equal the other. I know It's something really simple I am missing, but can't figure it out.
     
  2. jcsd
  3. Jun 13, 2009 #2

    Dick

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    Clearly, you meant sin(x)^2-sin(y)^2. Use the sum formula for sin(a+b) and sin(a-b) and expand it. Then use cos^2-1=sin^2. Expand again.
     
  4. Jun 14, 2009 #3
    You are correct, I did mean sin2(x) - sin 2(y). And you clearly meant 1-cos2=sin2.
    Thanks.
     
  5. Jun 14, 2009 #4

    Dick

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    Right.
     
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