同学们都知道，两条平行线被第三条直线所截，同位用相等，内错角相等．由此可知．平行线具有迁移角的功能，即不改变角的大小将它从一个位置迁移到另一个位置．利用平行线的这一功能，我们可以给出三角形内角和定理的另外两种证法．已知：．求立之分析一要证三个角的和等于180 As the classmates all know, the two parallel lines are truncated by the third straight line, and the same position is equal and the internal angle is equal. This shows that. The parallel line has the function of migrating angle, ie it is moved from one position to another without changing the size of the angle. Using this function of parallel lines, we can give two other proofs of triangle inner angle and theorem. A known:. The analysis of the stand is to prove that the sum of the three angles is equal to 180