# Math Help - Geometry Proof

1. ## Geometry Proof

OB=arcOA= $\theta$

Prove that OP= 1-cos $\theta$

Deduce that OD = $\frac{\theta(1-cos\theta)}{\theta-sin\theta}$

thanks.

2. is it known that the circle's center lies on the line OP? Is C the center of the circle? If yes, it's pretty obvious that $CP = \cos\theta$.

For the second equality, let A' be the projection of A on OB. Use the fact that the triangle BA'A is similar to the triangle BOD.

3. yes to your questions, but why cp=cos theta
i cant see it, im stupid
and can you explain the 2nd part in more details.

thanks.

4. Originally Posted by BabyMilo
why cp=cos theta
Because $\angle OCA=\theta$. The fact that $CP=\cos\theta$ follows from the definition of cosine.

and can you explain the 2nd part in more details.
Draw a horizontal line through A and call its intersection with the line OB by A'. Then the triangle BA'A is similar to the triangle BOD because all three angles are equal. Therefore the ratio of A'A = OP to OD is the same as the raio of BA' to BO.