In euclidian geometry a cyclic quadrilateral or iniscribed quadrilateral is a quadrilateral whose vertices all lie on a single circle . this circle is called circumcircle or circumscribed circle and the vertices are said to be cyclonic.
the formula to calculate area of cyclic quadrilateral is :-
A = (s-a) (s-b)(s-c)(s-d) in underoot.
hope this makes u understand.