三角形是最基本的几何图形之一 ,它的三边相互制约 ,三内角之和为定值 .同时 ,它的边角之间有着密切的联系 (如大角对大边 ,大边对大角 ,正、余弦定理等 ) ,面积又可以用边和角来表示 ,因而在初中数学竞赛中备受命题者的青睐 .一、基础知识1 一般三角形的边满足ａ +ｂ ｃ,ａ The triangle is one of the most basic geometric shapes. Its three sides are mutually restricted, and the sum of the three inner angles is a fixed value. At the same time, there are close relations between its corners (such as large angle to large scale, large scale to large angle, positive The cosine theorem, etc.) The area can be expressed in terms of edges and angles, and is thus favored by the propositioners in the junior high school math competition. I. Basic knowledge 1 The sides of a general triangle satisfy a +b c,a