本文从寻求过定点的动弦在运动过程中的不变量来导出相交弦定理和切割线定理。这样处理教材既有利于揭示问题的实质和便于将它们统一为圆幂定理,又有助于激发学生对数学的美感,培养学生的探究能力。 This paper derives the theorem of chord crossing and cutting line from the invariants of the fixed-point moving chord in the course of movement. This teaching material not only helps to reveal the substance of the problem and facilitate the unification of them into a round power theorem, but also helps to arouse students 'aesthetics of mathematics and cultivate students' inquiry ability.