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Homework Help: How to prove this identity?

  1. Jan 24, 2005 #1
    :mad: how would u proove this identity

    [tex] \cos^2 \theta= \cos^2 - \sin^2= 2\cos^2\theta-1=1-2\sin^2\theta [/tex] ? The example i was given was [tex] \sin2\theta=2\sin\theta \cos\theta [/tex] this was a little bit easier to understand since there was a left and right side, im not sure how to proove this. How was this formula derived from the sum differnce formula?
     
  2. jcsd
  3. Jan 24, 2005 #2
    cos(a+b) = cos(a)cos(b)-sin(a)sin(b)
    therefore
    cos(2a)=cos(a)cos(a)-sin(a)sin(a)
    this is the first step... the rest should be easy
     
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