三角形是最基本的几何图形之一,它是研究其他复杂图形的基础.三角形的三边互相制约,三个内角之和为定值,边与角之间也有密切的联系(如大角对大边,大边对大角等,以后将会学到).三角形边、角间的基本关系有三角形三边关系定理,三角形内角和定理.它们在线段、角度的计算以及图形的计数等方面有着广泛的应用. The triangle is one of the most basic geometric shapes. It is the basis for the study of other complex shapes. The three sides of the triangle restrict each other, the sum of the three interior angles is fixed, and there is also a close connection between the sides and the corners (such as the large to the large side. The major relations between triangle edges and corners are the triangular trilateral relations theorem, triangle internal angles and theorems. They have a wide range of aspects such as line segment, angle calculation, and graph counting. application.