同心圆问题在近几年的中考试题中屡见不鲜 .由于两圆的特殊位置关系 ,使得图形中的几何元素有着许多重要的性质 ,从而为相交弦定理、切线长定理、切割线定理以及垂径定理、勾股定理等的应用提供了用武之地 .图 1一、求线段的积例 1　如图 1 ,已知两个同心圆 ,其中大圆的半径为 The problem of concentric circles is common in middle school examination questions in recent years. Due to the special positional relationship between the two circles, the geometric elements in the graph have many important properties, which are intersecting string theorem, tangent length theorem, cutting line theorem and vertical trail theorem. The application of Pythagorean theorem and other theorems provide a place to use. Fig. 1 I. Example of finding the line segment 1 As shown in Fig. 1, two concentric circles are known, in which the radius of the great circle is