Two points define a straight line. The line segment between two points is the shortest. One and only one straight line is perpendicular to the known straight line.

I want to know the meaning of axioms, theorems and proofs, as well as their relations and differences with propositions; I will distinguish the propositions and conclusions of axioms and theorems, and write the propositions in the form of "if so"; I will realize the necessity of using examples to prove, cultivate a good sense of expressing ideas in a coherent way, understand the steps and formats of proof, and learn to focus on what are axioms, theorems and proofs. Learning difficulties: Understand the steps and format of proof, and experience the rigor of proof. Each proposition consists of two parts. Provided that the conclusion is general, all propositions can be written in the form of "if" and "then".