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解答与圆有关的几何问题时,常常需要添加辅助线,以便迅速打通解题思路．在解决有关两圆相切的问题时,公切线是解决问题的关键．当题目的已知条件中有两圆相切时,首先考虑过切点作两圆的公切线,以便利用弦切角定理等其他有关的定义、定理、性质来沟通两圆的关系,为解决问题提供新的条件．下面举例说明．
When solving geometry problems related to circles, it is often necessary to add auxiliary lines so that they can quickly solve problems. In solving the problem concerning the tangency of two circles, the public tangent is the key to solving the problem. When there are two circles tangent in the known condition of the topic, first consider the tangent point as the common tangent of the two circles in order to use the other related definitions, theorems, and properties of the chord angle theorem to communicate the relationship between the two circles. The problem provides new conditions. Here is an example.