1 + 1 = 2
Base case: 1 + 0 = 1, by the first recursive definition.
Induction step: Assume that 1 + n = n + 1, for some natural number n. Then, 1 + (n + 1) = (1 + n) + 1, by the second recursive definition. By the induction hypothesis, this is equal to (n + 1) + 1. Using the commutativity of addition (which can be proved from the Peano axioms), we can write this as n + (1 + 1), which is equal to n + 2 by the first recursive definition. Therefore, 1 + (n + 1) = n + 2, and the proof is complete.
Therefore, we have shown that 1 + 1 = 2, using the Peano axioms and mathematical induction.
Ha hope this helps :D