三角形内角和定理的证明,课本已给出了一种证法,此定理是添辅助线证明的第一例,本文着重谈谈证明思路的选择途径. 已知:如图1,△ABC. 求证:∠A+∠B+∠C=180°. 思维过程:欲证三角形三个内角之和等于180°,联想我们学过的与180°有关的角有哪些呢?一是一个平角等于180°,二是两条平行线被第三 The proof of the triangle internal angle and theorem, the textbook has given a proof method, this theorem is the first example to add the auxiliary line proof, this article focuses on the way to prove the proof of choice. Known: Figure 1, △ ABC. Proof : ∠A+∠B+∠C=180°. Thinking process: If you want to prove that the sum of the three inner angles of the triangle is equal to 180°, what are the angles that we learned about 180°? The first is a flat angle equal to 180°. Is two parallel lines are third