在证明四条线段成比例时,经常会碰到要证的四条线段在同一直线上的情形,此时,不能直接应用平行线分线段成比例定理或相似三角形对应边成比例的性质去解决,而应采取代换方法,将共直线的线段成比例转化为不共直线的线段成比例.常见的代换方法有以下几种. When the four line segments are proved to be proportional, the four line segments to be proved are often encountered on the same straight line. At this time, the proportional proposition of the parallel line split line segment or the proportionality of the similar triangles cannot be directly used to solve the problem. Replacement method should be adopted to convert the line segments of a common line into proportions of line segments that are not common. The common substitution methods are as follows.