“圆内接四边形的两条对角线的乘积等于两组对边乘积之和”，这就是著名的托勒密定理．但在统编教材中它以习题的面目出现，不被人们所重视．笔者认为既是定理，就可作为推理论证的依据．有些题目如能灵活巧妙地运用它，则往往使解证过程简捷清新，收到事半功倍之效．兹举... “The product of the two diagonals of a circle inscribed in a quadrilateral is equal to the sum of two sets of opposite side products”, which is the famous Ptolemy theorem. However, in the textbook, it appeared in the face of exercises and was not taken seriously by people. The author believes that it is a theorem and it can be used as a basis for deduction of theoretical evidence. Some topics such as the ability to use it flexibly, it often makes the verification process simple and fresh, and get twice the result with half the effort. To give ...