等积式转化成比例式是证等积式的一个重要思维过程,转化成比例式后,要证四条线(或三条)成比例,可证两个三角形相似。到底证那两个三角形相似,图形简单的可以直接观察。图形复杂点的,需添设辅助线的,学生往往不知从何下手。为了突破这一难点,在教学中重点帮助学生掌握:“横看、竖看一组三角形相似”的方法。这种方法对等积式、比例式的绝大部份题都适用。特以这几年考试题为例说明这个问题。例1 如图,已知弦AB,CD相交于P,连BD,CA,并延长相交 The conversion of isomorphism to proportionality is an important thinking process of the syndrome isomorphism. After conversion into a proportional model, it is necessary to prove that the four lines (or three lines) are proportional to each other and that the two triangles are similar. In the end, the two triangles are similar, and the graphic can be directly observed. In the case of complex graphics, supplementary lines are needed. Students often do not know where to start. In order to break through this difficulty, in the teaching focus on helping students master: “viewing horizontally and vertically viewing a group of similar triangles” method. This method is applicable to most of the equations and proportional equations. Special exam questions for the past few years illustrate this issue. Example 1 As shown in the figure, the strings AB, CD are known to intersect at P, connect BD, CA, and extend the intersection