到一个角的两边的距离相等的点,在这个角的平分线上,这就是角平分线的判定定理,遗憾的是这一定理常被同学们忽视,其实,应用这一定理证明角平分线问题,显得特别简单. 例1 如图1,已知∠ABD=∠ACD=90°,∠1=∠2.求证:AD平分∠BAC. 证明因为∠1=∠2,所以DB=DC, The points on both sides of an angle are equal, on the bisector of this angle, this is the decision theorem of the angle bisector. Unfortunately, this theorem is often overlooked by students. In fact, the theorem is used to prove the angle bisector. The problem is particularly simple. Example 1 As shown in Figure 1, it is known that ABD = ∠ ACD = 90°, ∠ 1 = ∠ 2. Proof: AD bisects ∠ BAC. Prove that because ∠ 1 = ∠ 2, so DB = DC,