几何“ａ2 =ｂｃ”型的命题 ,综合性强 ,证法灵活 ,是训练初中学生思维能力的重要题型 ,也一直是中考的“热点” .本文举例说明此类题型常用的证明方法利用共边相似三角形证明　　　　　图 1例 1　如图 1,已知⊙Ｏ与⊙Ａ相交于Ｂ、Ｃ两点 ,经过点Ａ ,过Ａ作⊙Ｏ的弦ＡＦ交⊙Ａ The proposition of geometry type “a2 =bc”, which is comprehensive and flexible, is an important question for training junior high school students’ thinking ability, and has always been a “hot spot” for the middle school entrance examination. This article illustrates the use of the commonly used proof methods for such questions. Co-edge similarity triangle proved Figure 1 Example 1 As shown in Figure 1, it is known that ⊙O intersects with ⊙A at two points B and C, passes point A, and passes through A for 弦O’s chord AF.