我们知道，三角形的外角有这样的性质：三角形的一个外角等于与它不相邻的两个内角的和，大于任何一个与它不相邻的内角．这个性质是研究三角形的重要基础知识，应用也非常广泛．现分类举例说明．一、计算角度例１如图１，Ｄ是△ＡＢＣ中ＣＢ的延长线上一点，ＤＯＡＢ于０，Ｃ We know that the outer corner of a triangle has the property that an outer corner of a triangle is equal to the sum of the two inner corners that are not adjacent to it, and is larger than any inner corner that is not adjacent to it. This property is an important basic knowledge for the study of triangles and its application is very extensive. Classified as an example. First, the calculation of angle Example 1 in Figure 1, D is △ ABC in the extension of CB, DOAB in 0, C