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三角形的外角有两条性质: 1．三角形的一个外角等于与它不相邻的两个内角的和; 2．三角形的一个外角大于任何一个与它不相邻的内角．这两条性质都表明了三角形的外角与内角之间的一种关系．第1条性质常常用于在几何图形中寻找角与角之间的相等关系;第2条性质常常用于证明角与角之间的不等关系(大小关系)．可以用三角形外角性质解答的题目通常都存在共同的特征,下面通过两个具体的例子来总结其中的规律．
The triangle’s outer corner has two properties: 1. An outer angle of a triangle is equal to the sum of the two inner angles that are not adjacent to it; An outer corner of a triangle is larger than any inner corner that is not adjacent to it. Both of these properties indicate a relationship between the outer corner and the inner corner of a triangle. The first one is often used to find the equal relationship between angles and angles in geometry; the second one is often used to prove the inequality (size relationship) between angles. The problems that can be solved by using the triangular triangles outside the corners usually have common characteristics. The following two examples are used to summarize the rules.