同学们都已经知道三角形的内角和为180° ,但你是否想过除了课本的证明方法外 ,还有没有其它的证明方法呢 ?下面我们就来探讨三角形内角和定理的多种证明方法 .已知△ABC ,求证 :∠A +∠B +∠C=180° .证明一　常见的证法 ,过点C作CE∥AB ,延长BC至D ,则∠A +∠B +∠C =∠ 1+∠ The students have already known that the triangle’s internal angle is 180°, but have you ever wondered if there is any other proof method besides the textbook proof method? Let’s discuss the various proof methods of the triangle internal angle and the theorem. Know △ABC, verify: ∠A + ∠B + ∠C = 180°. Prove a common proof, overpoint C for CE∥AB, extend BC to D, then ∠A +∠B +∠C =∠ 1 +∠