-- Mathlib/Algebra/Polynomial/Div.lean
theorem map_mod_divByMonic [Ring S] (f : R →+* S) (hq : Monic q) :
    (p /ₘ q).map f = p.map f /ₘ q.map f ∧ (p %ₘ q).map f = p.map f %ₘ q.map f

Proof state:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
⊢ map f (p /ₘ q) = map f p /ₘ map f q ∧ map f (p %ₘ q) = map f p %ₘ map f q

Tactic:
nontriviality S

Result:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
a✝ : Nontrivial S
⊢ map f (p /ₘ q) = map f p /ₘ map f q ∧ map f (p %ₘ q) = map f p %ₘ map f q

Proof state:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
a✝ : Nontrivial S
this : Nontrivial R
⊢ map f (p /ₘ q) = map f p /ₘ map f q ∧ map f (p %ₘ q) = map f p %ₘ map f q

Tactic:
have : map f p /ₘ map f q = map f (p /ₘ q) ∧ map f p %ₘ map f q = map f (p %ₘ q) :=
  div_modByMonic_unique ((p /ₘ q).map f) _ (hq.map f)
    ⟨Eq.symm <| by rw [← Polynomial.map_mul, ← Polynomial.map_add, modByMonic_add_div _ hq],
      calc
        _ ≤ degree (p %ₘ q) := degree_map_le
        _ < degree q := degree_modByMonic_lt _ hq
        _ = _ :=
          Eq.symm <|
            degree_map_eq_of_leadingCoeff_ne_zero _
              (by rw [Monic.def.1 hq, f.map_one]; exact one_ne_zero)⟩

Result:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
a✝ : Nontrivial S
this✝ : Nontrivial R
this : map f p /ₘ map f q = map f (p /ₘ q) ∧ map f p %ₘ map f q = map f (p %ₘ q)
⊢ map f (p /ₘ q) = map f p /ₘ map f q ∧ map f (p %ₘ q) = map f p %ₘ map f q

Proof state:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
a✝ : Nontrivial S
this : Nontrivial R
⊢ f q.leadingCoeff ≠ 0

Tactic:
rw [Monic.def.1 hq, f.map_one]

Result:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
a✝ : Nontrivial S
this : Nontrivial R
⊢ 1 ≠ 0

Proof state:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
a✝ : Nontrivial S
⊢ map f (p /ₘ q) = map f p /ₘ map f q ∧ map f (p %ₘ q) = map f p %ₘ map f q

Tactic:
haveI : Nontrivial R := f.domain_nontrivial

Result:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
a✝ : Nontrivial S
this : Nontrivial R
⊢ map f (p /ₘ q) = map f p /ₘ map f q ∧ map f (p %ₘ q) = map f p %ₘ map f q

Proof state:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
a✝ : Nontrivial S
this : Nontrivial R
⊢ map f p = map f (p %ₘ q) + map f q * map f (p /ₘ q)

Tactic:
rw [← Polynomial.map_mul, ← Polynomial.map_add, modByMonic_add_div _ hq]

Result:
no goals

Proof state:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
a✝ : Nontrivial S
this : Nontrivial R
⊢ 1 ≠ 0

Tactic:
exact one_ne_zero

Result:
no goals

Proof state:
R : Type u
S : Type v
inst✝¹ : Ring R
p q : R[X]
inst✝ : Ring S
f : R →+* S
hq : q.Monic
a✝ : Nontrivial S
this✝ : Nontrivial R
this : map f p /ₘ map f q = map f (p /ₘ q) ∧ map f p %ₘ map f q = map f (p %ₘ q)
⊢ map f (p /ₘ q) = map f p /ₘ map f q ∧ map f (p %ₘ q) = map f p %ₘ map f q

Tactic:
exact ⟨this.1.symm, this.2.symm⟩

Result:
no goals