中学所学的平面几何源于欧几里得《几何原本》 ,而欧氏几何的基础有五个公设和五大公理 .其中的第五公设说“若一直线与两直线相交 ,且若同侧所交两内角之和小于两直角 ,则两直线无限延长后必相交于该侧的一点 .”现在几何书上的平行线公理就是由此而来 .从公元前 3 0 0年直到 The plane geometry learned in middle school is derived from Euclid’s Geometry Original, and the basis of Euclidean Geometry consists of five quintessions and five axioms. The fifth communicator says, “If a straight line intersects two straight lines, and if the same side The sum of the two internal angles crossed is less than two right angles, and then the two lines will extend indefinitely and will intersect at one point on that side.” Now the parallel line axioms in the geometry book come from it. From 300 BC until