三角形内角和定理 (以下简称定理 )是学生在初中几何中碰到的第一个证明难度较大的几何定理 .它既是重点 ,也是难点 .学生学习中存在的困难主要有 :( 1)该定理证明是初二几何第一个正规的证明 ,且证明过程较长 ;( 2 )学生对几何证明还比较生疏 ;( 3)第一次正式添加辅助线 .若要 Triangle inner angles and theorems (theorems) are the first geometrical theorem that students find difficult to meet in junior high school geometry. It is both important and difficult. Students have difficulties in learning: (1) The theorem The proof is the first formal proof of the first two geometry, and the proof process is long; (2) The student is still relatively unfamiliar with the geometric proof; (3) The first formal addition of auxiliary lines.