初中几何第二册第五章第二节“平行线分线段成比例定理”，是研究相似形最重要和最基本的理论。课本对这个定理是用举例的方法引入的，没有给予严格的证明，人教社编写的配套用书《教师教学用书》对该定理没有给出严格证明作了如下解释：“因为证明涉及无理数理论、极限思... The second chapter of the second chapter of the junior high school geometry, the “proportional theorem of the parallel line segment”, is the most important and basic theory for studying similarity. The textbook introduces the theorem by way of example. It does not give strict proofs. The supporting book “Teacher’s Teaching Book” written by the People’s Education Agency makes the following explanation for the theorem: “Because the proof involves irrational number theory, Extreme thinking...