在平面几何中,有两组“巧然相似”的几何定理!——究竟是哪两组呢?为了使该组定理的表述更为简洁,我们约定本文中的几个常用概念,其内涵和外延如下:平行线——两条直线没有公共点;两条或两条以上的一组直线互相平行,称为平行线组,一组平行线至少是两条直线.如果没有特别说明,本文中的平行线——即指两条平行直线. In plane geometry, there are two sets of “theoretically similar” geometrical theorems! - Which of these two groups? In order to make the formulation of the set of theorems more concise, we agree on several commonly used concepts in this article, which Connotation and denotation are as follows: Parallel lines - two lines have no common point; two or more than one set of lines parallel to each other, called parallel lines, a group of parallel lines at least two straight lines.If not otherwise specified, The parallel lines in this article - that is, two parallel straight lines.