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Proving half angle formulae

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

    Prove that cos([tex]\frac{\theta}{2}[/tex]) = [tex]\pm[/tex][tex]\sqrt{\frac{1+cos\theta}{2}}[/tex] using the unit circle.

    2. Relevant equations

    3. The attempt at a solution

    I'm not sure if it's possible for you to provide a clear graph on here for the solution but a link would also be nice :)

    View attachment half angle.bmp
  2. jcsd
  3. Oct 26, 2009 #2


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    Homework Helper

    consider the points A=(cos(t),sin(t)) B=(cos(t/2),sin(t/2)) C=(1,0) D=(0,0)
    To find B show that the midpoint of the segment AC is on the segment BD
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