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三角形的内角和等于180°,这一定理证法多种多样,但其基本思路有二,一是构造平角形成180°,二是构造平行线形成同旁内角互补从而构成180°.一、构造平角方法一:如图1,延长BC至D,过C作射线CE∥BA,则∠1=∠A(两直线平行,内错角相等)∠2=∠B(两直线平行,同位角相等)(下略).方法二:如图2过点B作MN∥AC则∠ABN=∠A,∠CBM=∠C.(两直线平行,内错角相等)(下略)方法三:如图3,在BC上任意取点P,过P作PE∥AC、DF∥AB分
The internal angle of the triangle is equal to 180°. The proof of this theorem is varied. However, there are two basic ideas: one is that the structural flat angle forms 180°, and the other is that the structural parallel lines form complements with the internal angle and form 180°. Straight angle method one: as shown in Figure 1, extend BC to D, over C for radiation CE∥BA, then ∠1 = ∠A (two straight lines, the inner angle is equal) ∠2 = ∠B (two straight lines, equal angle is equal ) (Slightly omitted). Method 2: as shown in Figure 2 over point B for MN ∥ AC then ∠ ABN = ∠ A, ∠ CBM = ∠ C. (two straight lines, within the same angle) (slightly omitted) Method Three: If Fig. 3, point P is arbitrarily taken on BC, P is used as PE∥AC, DF∥AB is divided