在解有关两圆相交或相切的问题时,公共弦或公切线是常用的辅助线,通过它们可把分散在两圆上的角转化到同一圆上或同一顶点处(切点、公共弦端点),从而把未知的、不熟悉的两圆上的角的关系转化为已知的、熟悉的同圆上的圆心角、圆质角、弦切角的关系,使问题得到解决. When solving the problem of intersection or tangency of two circles, a common string or common tangent is a common auxiliary line through which the angles scattered on two circles can be transformed into the same circle or at the same vertex (cut point, public string (Endpoints), so that the relationship between the unknown, unfamiliar angles on the two circles is transformed into the known, familiar relationship of the circle angle, circle angle, and string cut angle on the same circle, so that the problem is solved.