同学们知道,等腰三角形底边上中线、高线及顶角的角平分线是互相重合的,我们把等腰三角形的这一性质简称为“三线合一”。一、利用“三线合一”的性质寻找证题途径例1已知:如图1,在△ABC中,AB=AC,BD是AC边上的高。求证:∠CBD=21∠A图1分析:当题目中有等腰三角形的已知条件时,常 The students know that the bisectors of the midline, highline, and vertex angles on the base of the isosceles triangle coincide with each other. We call this feature of isosceles triangles “three-in-one”. First, the use of the “three-in-one” nature to find evidence approach Example 1 is known: Figure 1, in △ ABC, AB = AC, BD is high on the AC side. Proof: ∠CBD=21∠A Figure 1 Analysis: When there is a known condition of isosceles triangle in the title, often