-- Mathlib/RingTheory/Polynomial/RationalRoot.lean
theorem exists_integer_of_is_root_of_monic {p : A[X]} (hp : Monic p) {r : K} (hr : aeval r p = 0) :
    ∃ r' : A, r = algebraMap A K r' ∧ r' ∣ p.coeff 0

Proof state:
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
⊢ ∃ r', r = (algebraMap A K) r' ∧ r' ∣ p.coeff 0

Tactic:
obtain ⟨inv, h_inv⟩ := hp ▸ den_dvd_of_is_root hr

Result:
case intro
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
⊢ ∃ r', r = (algebraMap A K) r' ∧ r' ∣ p.coeff 0

Proof state:
case right
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
h : inv ∣ 1
⊢ num A r * inv ∣ p.coeff 0

Tactic:
simpa using mul_dvd_mul (num_dvd_of_is_root hr) h

Result:
no goals

Proof state:
case intro
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
⊢ ∃ r', r = (algebraMap A K) r' ∧ r' ∣ p.coeff 0

Tactic:
use num A r * inv, ?_

Result:
case right
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
⊢ num A r * inv ∣ p.coeff 0

case left
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
⊢ r = (algebraMap A K) (num A r * inv)

Proof state:
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
⊢ 1 = inv * ↑(den A r)

Tactic:
simpa [mul_comm] using h_inv

Result:
no goals

Proof state:
case left
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
d_ne_zero : (algebraMap A K) ↑(den A r) ≠ 0
⊢ r = (algebraMap A K) (num A r * inv)

Tactic:
nth_rw 1 [← mk'_num_den' A r]

Result:
case left
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
d_ne_zero : (algebraMap A K) ↑(den A r) ≠ 0
⊢ (algebraMap A K) (num A r) / (algebraMap A K) ↑(den A r) = (algebraMap A K) (num A r * inv)

Proof state:
case right
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
⊢ num A r * inv ∣ p.coeff 0

Tactic:
have h : inv ∣ 1 := ⟨den A r, by simpa [mul_comm] using h_inv⟩

Result:
case right
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
h : inv ∣ 1
⊢ num A r * inv ∣ p.coeff 0

Proof state:
case left
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
d_ne_zero : (algebraMap A K) ↑(den A r) ≠ 0
⊢ (algebraMap A K) (num A r) / (algebraMap A K) ↑(den A r) = (algebraMap A K) (num A r * inv)

Tactic:
rw [div_eq_iff d_ne_zero, map_mul, mul_assoc, mul_comm ((algebraMap A K) inv),
  ← map_mul, ← h_inv, map_one, mul_one]

Result:
no goals

Proof state:
case left
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
⊢ r = (algebraMap A K) (num A r * inv)

Tactic:
have d_ne_zero : algebraMap A K (den A r) ≠ 0 :=
  IsFractionRing.to_map_ne_zero_of_mem_nonZeroDivisors (den A r).prop

Result:
case left
A : Type u_1
K : Type u_2
inst✝⁵ : CommRing A
inst✝⁴ : IsDomain A
inst✝³ : UniqueFactorizationMonoid A
inst✝² : Field K
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
p : A[X]
hp : p.Monic
r : K
hr : (aeval r) p = 0
inv : A
h_inv : 1 = ↑(den A r) * inv
d_ne_zero : (algebraMap A K) ↑(den A r) ≠ 0
⊢ r = (algebraMap A K) (num A r * inv)