Proving half angle formulae

[Louis B]

1. The problem statement, all variables and given/known data

Prove that cos(\frac{\theta}{2}) = \pm\sqrt{\frac{1+cos\theta}{2}} 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 :)

21351

[lurflurf]

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