# Proving half angle formulae

1. Oct 26, 2009

### 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 :)

View attachment half angle.bmp

2. Oct 26, 2009

### 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