A theorem is a statement that can be proved from the axioms and other theorems by logical argument.
We have also encountered some theorems in Section A, Units 3 and 11, and Section B, Unit 15. For example: Theorem 1: Vertically opposite angles are equal in measure.
A proof is a step-by-step explanation that uses axioms and previously proven theorems to show another theorem is correct.
A proof is written as a series of statements accompanied by the reason why each statement is true.
If you use a piece of information that you have been told in the question as a statement, the reason is stated as ‘given’.
For example, in Section A, Unit 3 you met the proof that vertically opposite angles are equal. We will look at it again here to show the structure of a proof.