# Thread: Circle Geometry: Determine Size of Angle in Terms of Unknown

1. ## Circle Geometry: Determine Size of Angle in Terms of Unknown

A chord BC and the diameter from A meet at a point P such that OC = CP. If angle CPO = a, prove that angle AOB = 3a. This is a method of trisection an angle by construction.

What does this trisection of angle mean? Are there any other methods of solution?

2. Let D be the other point of intersection between AP and the circle.

We have $\displaystyle m(\widehat{AOB})=m(arcAB)=x$

Then:

$\displaystyle a=m(\widehat{BPA})=\displaystyle\frac{m(arcAB)-m(arcCD)}{2}=\displaystyle\frac{x-a}{2}$

$\displaystyle a=\displaystyle\frac{x-a}{2}\Rightarrow 2a=x-a\Rightarrow x=3a$

3. Sorry, i am just not seeing it. Mabey its the notations, that i am not use to.

Can you just tell me what arcCD represents? Is it subtended at centre or circumference.
And i can guess that m is the angle size?

And also, the 2nd line, containing m(arc AB) - m(arc CD), what rule is that? Just point me in the right direction?