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Trigonometric identity

  1. Nov 27, 2008 #1
    Why is [tex]cos^2 x = \frac{1}{2} + \frac{cos(2x)}{2}[/tex]

  2. jcsd
  3. Nov 27, 2008 #2


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    why is
    why is
  4. Nov 27, 2008 #3


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    Starting where? Do you know cos(a+ b)= cos(a)cos(b)- sin(a)sin(b)? If so then take a= b= x. If not then what definition of cos(x) are you using so that you can get that?
  5. Nov 27, 2008 #4
    I think this is what you wanted to know:
    Since cos(2x)=cos2x-sin2x
    cos2x=1/2 + cos(2x)/2
  6. Nov 27, 2008 #5

    [tex]\frac{1}{2} + \frac{cos(2x)}{2}[/tex]





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