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说明:上图中的五个定理,是以相交弦定理为基础,用旋转的方法改变两弦交点的位置及弦的位置、用运动的观点找到它们之间的联系的。具体操作方法是: 相交弦定理中的弦AB绕点A逆时针旋转,同时两弦的交点P沿CD移动,当两弦的交点P在O外时(点P是两弦的外分点),得到割线定理,即PA·PB=PC·PD。割线ABP绕点P顺时针(或逆时针)旋转,使点A、B重合,这时PA是O的切线
Explanation: The five theorems in the above figure are based on the intersecting string theorem, using the rotation method to change the position of the two strings and the position of the strings, and find the connection between them with the view of motion. The specific method of operation is: The string AB in the intersecting string theorem rotates counterclockwise around the point A, and the intersection P of the two strings moves along the CD when the intersection P of the two strings is outside O (the point P is the outer point of the two strings) The secant theorem is obtained, that is, PA·PB=PC·PD. The secant ABP rotates clockwise (or anti-clockwise) around the point P so that points A and B coincide. At this time, PA is the tangent of O.