What are some properties of cyclic quadrilaterals? I've search Google, but the answers are too vague and not what I've had in mind.
(I'm using this for a proof)
Have you read the Wiki article on cyclic quadrilaterals? It gives a lot of information on the subject.
If it still does not answer your question, you could post your question and we could all learn something.
http://en.wikipedia.org/wiki/Cyclic_quadrilateral