学习平面几何,如果能积累一些重要的、常见的基本图形,熟悉它们的有关性质,对开拓解题思路,提高证题技巧是大有益处的.初中几何(人教版)第三册有这样一道题:题目如图1,⊙O1和⊙O2外切于点A,BC是⊙O1和⊙O2的公切线,B、C为切点.求证:AB⊥AC.证明过点A作⊙O1和⊙O2的内公切线交BC Learning plane geometry, if you can accumulate some important and common basic graphics, become familiar with their relevant nature, and it is of great benefit to develop problem-solving ideas and improve the skills of test questions. The third volume of junior high school geometry (PEP) has this One question: The title is shown in Figure 1. ⊙O1 and ⊙O2 are circumscribed at point A. BC is the common tangent line between ⊙O1 and ⊙O2. B and C are tangent points. Proof: AB⊥AC. Prove that point A is ⊙O1. ⊙ 2 2 2 2 2 内 内 BC BC