Theorem 2: Commutativity of addition:
Proof: Base case: By lemma 1, .
- Inductive hypothesis: Suppose that, for
,
.
- By axiom 6,
.
- By theorem 1,
.
- By the inductive hypothesis,
.
- By theorem 1,
.
- By lemma 1,
.
- By theorem 1,
.
- By axiom 6,
.
- By induction,
. QED.