在数学试题中,经常出现一类几何问题,从表面上看这类题与圆无关,但如果我们根据题目中的已知条件添加辅助圆,再利用圆的有关性质来解决问题,往往能起到化隐为显、化难为易、化繁为简的解题效果.下面以中考题为例,说明以下几种情形时宜用辅助圆.一、当有公共端点的等线段时例1如图1,在△ABC内有一点D,且DA=DB=DC,若∠DAB=20°,∠DAC= In mathematics test questions, there often appears a kind of geometric problem. On the surface, such questions have nothing to do with circles. However, if we solve the problem by adding the auxiliary circle according to the known conditions in the subject and then using the relevant properties of the circle, To the hidden potential is obvious, the difficulty is easy, simplification of the problem-solving effect. The following to the exam questions, for example, the following situations should be used when the auxiliary circle. 1, there is a little D in △ ABC, and DA = DB = DC, if DAB = 20 °, DAC =