Question: if abcd is a cyclic quadrilateral in which?
Answer: Intersection of diagonals (2) ABCD is a cyclic quadrilateral if and only if points P and Q are the midpoints of sides AB and CD. The intersection P may be internal or external to the circle. In the former case, the cyclic quadrilateral is ABCD, and in the latter case, the cyclic quadrilateral is ABDC.