对于线段的和差倍分在几何论证中视为简单的一类 ,但方法选得不当其证明常带来困难 ,这里介绍利用截长补短 ,加倍折半法转化为证明线段的相等 ,特举例说明。例 1　在梯形ＡＢＣＤ中 ,ＡＢ∥ＤＣ ,ＡＤ⊥ＡＢ ,∠ＢＣＤ平分线ＣＭ过ＡＤ的中点Ｍ ,求证 :ＡＢ +ＣＤ =ＢＣ。图 (1 For line segments and differential multiples, they are regarded as simple in geometric argumentation. However, improper selection of methods often brings difficulties. Here we introduce the use of truncating and shortening, doubling the half method into the equality of line segments, and giving examples to illustrate . Example 1 In the trapezoidal ABCD, AB∥DC, AD⊥AB, and ∠BCD bisect the line CM through the middle point of AD to verify: AB +CD =BC. figure 1