The circles of Apollonius are formed by two pencils: The first is the ratio r of the distances from one point P to two fixed points C and D, denoted as follows: { P{x, y} | dPC / dPD = r } The second is the geometric locus of the P{x, y} that form an inscribed theta angle with the fixed points C and D, it is denoted as follows: { P{x, y} | angle CPD = theta }.
