I am looking for a formula/method to solve angular measuremnt problems in circles, any help of advising of a formula/method would be appreciated.

Example,

I have a circle, in the circle is a arc O to A and O to B, the arc is a minor arc. I know the length of the arc, and I know the length of the radius, what I want to know is the formula/method to work out the angle subtended by this arc?

I know the angle in the attachment is 90 degrees, but the angle in my problem is an obtuse angle in the circle. The attachment is for guidance only.

I have not given more information because I wish to solve the problem myself before anyone else solves it for me.

Thanks

David