6 The diagram, not drawn to scale, shows two concentric circles, centre $O$ .
$AB$ and $AD$ are tangents to the smaller circle at $B$ and $D$ respectively, centre $O$ .
$AC$ is the diameter of the larger circle. (a) Show that triangle $ABC$ is congruent to triangle $ADC$ .
Give a reason for each statement you make.

Worked solution

1. 由圆切线的对称性质找出对应边和对应角的相等关系

2. 根据全等判定定理得出结论

由 SAS（边角边）全等判定定理：
$\triangle ABC \cong \triangle ADC$

Answer

已证 $\triangle ABC \cong \triangle ADC$ （SAS）。

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