1: Preliminaries Last updated Save as PDF Page ID 23583 Anton Petrunin Pennsylvannia State University 1.1: What is the axiomatic approach? 1.2: What is a model? 1.3: Metric spaces 1.4: Shortcut for distance 1.5: Isometries, motions, and lines 1.6: Half-lines and segments 1.7: Angles 1.8: Reals modulo 2π 1.9: Continuity 1.10: Congruent triangles