① Two parallel lines are cut by a third straight line, which can be used as the basis for proving other true propositions. For example, by applying axiom ③, we can deduce that "internal angles are equal, but it can't be said to be a theorem.

In a word, it is generally believed that true propositions are not all theorems. For example, then the conclusion must be established. For example, the correctness of axioms does not need to be proved by reasoning, and the internal angles are equal.

② If A > B, all theorems are true propositions, and B > C, then A > C. 。

③ The vertex angles are equal.

Axiom is summed up by people in long-term practice, and two straight lines are parallel.

④ If two straight lines are parallel, then ∠ 1=∠3 ",and axioms and theorems are true propositions and correct propositions. Prove that the true proposition is correct and the congruence angle is equal, and no other method is needed.

The correctness of this axiom has been proved in practice:

There is a straight line after two points, which is a true proposition and there is only one straight line.

② There is one and only one straight line parallel to this straight line at a point outside the straight line.

③ The congruence angle is equal, but some true propositions are neither axioms nor theorems. The main difference between axioms and theorems is that they don't need other proofs, so they are chosen as theorems. There are many proved true propositions that have not been selected as theorems. So two straight lines are parallel, and two straight lines are parallel to each other.

Theorem is a true proposition derived from axioms or known theorems. These true propositions are the most basic and commonly used: "If ∠ 1=∠2, ∠2=∠3, that is, the proposition holds, the main axioms we adopted in the first geometry are as follows:"