证比例式(或等积式)和求线段的比是初中几何中的一类常见问题在不能直接应用相似三角形的性质,或没有平行线的情况下,作平行辅助线是解决这类问题的主要途径这里介绍一种作平行辅助线的常用方法:紧扣结论,补全图形,使其与基本图形(如图1,图2)相吻合,然后利用基本图形的相关性质求解.图1和图2很像大写字母A和X,我们不妨把它称作A形图和X形图.由A形图,可知DE∥BC, The ratio of cardinal (or isositive) and finding line segments is a common problem in junior high school geometry. Parallel lines can solve such problems if they can not be directly applied to the properties of similar triangles, or to parallel lines Main Approach Here we introduce a common method for parallel guides: concluding the conclusion, filling in the graph, fitting it to the basic graph (Figure 1, Figure 2), and then solving for the underlying graph’s basic properties. Figure 1 Figure 2 looks like the capital letters A and X. We might as well call it the A and X. From the A figure, we know that DE // BC,