![](http://dsideal.obs.cn-north-1.myhuaweicloud.com/HuangHai/BlogImages/2023/04/5265283b0c47a6aad5c85b0a0835994c.png)

- $\triangle ABF \cong \triangle AED$
- $\therefore \angle 2=\angle BAF$
- $\because \angle 3+\angle FAB=90^{\circ}$
- $\therefore \angle 2+\angle 3=90^{\circ}$
- 画出外接隐藏圆，以$O$为圆心
- 套路，满满的套路！找出一个三角形吧：$\triangle OGB$!,$BG>=OB-OG$!