Must Be Proved Before They Can Be Accepted As True
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Theorems are statements in geometry that require proof.
A proof uses postulates and theorems to prove some statement.
Yes. That is what theorems are for. Once proven, their results do not need to be justified again (except for exams).
There are many kinds of statement that are not theorems: A statement can be an axiom, that is, something that is assumed to be true without proof. It is usually self-evident, but like Euclid's parallel postulate, need not be. A statement need not be true in all circumstances - for example, A*B = B*A (commutativity) is not necessarily true for matrix multiplication. A statement can be false. A statement can be self-contradictory for example, "This statement is false".
A theorem is a statement or proposition which is not self-evident but which can be proved starting from basic axioms using a chain of reasoned argument (and previously proved theorems).