等腰三角形具有“三线合一”的性质:等腰三角形的顶角平分线、底边上的中线、底边上的高互相重合.如图1,在△ABC中,AB=AC,D是BC上一点.(1)如果∠1=∠2,那么AD⊥BC,BD=CD;(2)如果BD=CD,那么∠1=∠2,AD⊥BC;(3)如果AD⊥BC,那么∠1=∠2,BD=CD.上述性质中,共存在4个关系式:AB=AC,∠1=∠2,AD⊥BC,BD=CD.而改写后的每条性质都有两个条件,且都有一个条件是“AB=AC”.反过来,在关系式∠1=∠2,AD⊥BC, The isosceles triangle has the property of “three-in-one”: the vertex angle bisector of the isosceles triangle, the middle line on the bottom and the high one on the bottom coincide with each other as shown in Figure 1. In ABC, AB = AC, (1) If ∠1 = ∠2, AD BC BC, BD = CD; 21 = ∠2, AD ⊥ BC if BD = CD; (3) If AD ⊥ BC, then ∠1 = ∠2, BD = CD. Of the above properties, there are four relations: AB = AC, ∠1 = ∠2, AD ⊥ BC, BD = CD. There are two conditions, and one of the conditions is that “AB = AC”. In turn, in the relationship ∠1 = ∠2, AD ⊥ BC,