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证明线段成比例或等积式常用的方法是利用相似三角形.其基本思想是:先找出与所证的比例式中的线段有关的两个三角形,然后设法证明这两个三角形相似.因此正确寻找并证明相关的两个三角形相似是解决这类问题的关键.如何由比例式找出相关的三角形,这是同学们感到比较困难的问题.为了帮助同学们解决这一难点,本文介绍一种常用的方法——“三点定形法”.
A common method for proving the proportion of a line segment or an isopleth is to use similar triangles. The basic idea is to first find the two triangles that are related to the line segment in the proof ratio, and then try to prove that the two triangles are similar. Finding and proving that the related two triangles are similar is the key to solving this type of problem. How to find the relevant triangles by proportionality is a problem that the students feel is difficult. In order to help the students solve this difficulty, this article describes a Commonly used method - “three-point setting method.”