20 18. Four-point * * * circle is a theorem proving knowledge point in junior high school before. After the revision of 20 18 junior high school mathematics curriculum standard, the knowledge of four * * * circles was deleted from the textbook, which greatly reduced the difficulty of students' thinking and proof, and also reduced the difficulty of examination. If four points on the same plane are on the same circle, it is called a * * * circle, or simply a "four-point * * * circle".