切割线定理是初中几何的一个重要定理,在证明中有着广泛的应用,也是各地中考命题的热点.但在实际应用过程中,同学们往往因为只关注定理本身所列等积式的运用,而忽视定理证明过程中的“第三比”的作用,导致思路受阻.本文略举几例说明“第三比”在证题中的作用. The cutting line theorem is an important theorem of junior high school geometry. It has a wide range of applications in the proof, and is also a hot spot of the propositions in the local examinations. However, in the practical application process, the students often only pay attention to the application of isomorphisms listed in the theorem itself. Neglecting the role of the “third ratio” in the proof of the theorem leads to the idea being blocked. This article gives a few examples of the role of the “third ratio” in the proof.