Coq: How to prove max a b <= a+b?

I am unable to prove the simple logic max a b <= a+b using coq's tactics. How should I go about solving it? Below is the code that I worked on till now. s_le_n is proved but not mentioned here for the sake of simplicity.

Theorem s_le_n: forall (a b: nat),  a <= b -> S a <= S b.
Proof. Admitted.

Theorem max_sum: forall (a b: nat), max a b <= a + b.
Proof. 
intros.
induction a.
- simpl. reflexivity.
- rewrite plus_Sn_m. induction b.
  + simpl. rewrite <- plus_n_O. reflexivity.
  + rewrite <- plus_Sn_m. simpl. apply s_le_n. rewrite IHa.

Solution

Taking into account @re3el comment, we start from their "pen and paper proof":

if a>b max a b = a, a < a+b; else max a b = b, b < a+b

Let's now translate that into Coq! In fact, the first thing we need to do is case on the decidability of <, this is done using the le_lt_dec a b lemma. The rest is routine:

Require Import Arith.

Theorem max_sum (a b: nat) : max a b <= a + b.
Proof.
case (le_lt_dec a b).
+ now rewrite <- Nat.max_r_iff; intros ->; apply le_plus_r.
+ intros ha; apply Nat.lt_le_incl, Nat.max_l_iff in ha.
  now rewrite ha; apply le_plus_l.
Qed.

However, we can improve this proof quite a bit. There are various candidates, a good one using the stdlib is:

Theorem max_sum_1 (a b: nat) : max a b <= a + b.
Proof.
now rewrite Nat.max_lub_iff; split; [apply le_plus_l | apply le_plus_r].
Qed.

Using my library of choice [math-comp], you can chain the rewrites to get a more compact proof:

From mathcomp Require Import all_ssreflect.

Theorem max_sum_2 (a b: nat) : maxn a b <= a + b.
Proof. by rewrite geq_max leq_addl leq_addr. Qed.

In fact, on the light of short proof, maybe the original lemma was not even needed in the first place.

edit: @Jason Gross mentions another style of proof a more seasoned used would use:

Proof. apply Max.max_case_strong; omega. Qed.

However, this proof involves the use of a heavyweight automation tactic, omega; I strongly advise all beginners to avoid such tactics for a while, and learn how to do proofs more "manually". In fact, using any of the SMT-enabled tactics, the original goal can be simply solved with a call to a SMT.