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I can't figure it out

  1. Feb 5, 2006 #1
    I have this trig identity problem i need to solve

    cos2x=cosx

    I have to find all the values of x, and i know the answer is x=0°+120°, but i can figure ou how. I've tried it over and over and my answer turns out to x=0°=180°. Please help, thank you.:uhh:
     
  2. jcsd
  3. Feb 6, 2006 #2

    Tide

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    Science Advisor
    Homework Helper

    First, 180 is NOT equal to 0. Second, 180 degrees is obviously not a solution (substitiute back into the equation to see this).

    HINT: [tex]\cos 2x = \cos^2 x - \sin^2 x[/tex]
     
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