1995 APMO 第 1 题

Let $PQRS$ be a cyclic quadrilateral such that the segments $PQ$ and $RS$ are not parallel. Consider the set of circles through $P$ and $Q$ , and the set of circles through $R$ and $S$ . Determine the set $A$ of points of tangency of circles in these two sets.

令 $PQRS$ 为循环四边形，使得线段 $PQ$ 和 $RS$ 不平行。考虑通过 $P$ 和 $Q$ 的圆集，以及通过 $R$ 和 $S$ 的圆集。确定这两个集合中圆的切点的集合$A$。