What type of quadrilaterals have a circumcircle? How would I know that such a quadrilateral is circumscribable? This would help in using brahmgupta's formula on math competitions.
Cyclic quadrilaterals are quadrilaterals that have all four vertices on a circle. You can know quite a lot of information about them, such as Ptolemy's Theorem and Brahmagupta's formula. I haven't had the opportunity to learn about this as much as I liked, but I hope this gets you started.