-- Mathlib/Tactic/NormNum/Basic.lean
theorem isRat_mul {α} [Ring α] {f : α → α → α} {a b : α} {na nb nc : ℤ} {da db dc k : ℕ} :
    f = HMul.hMul → IsRat a na da → IsRat b nb db →
    Int.mul na nb = Int.mul k nc →
    Nat.mul da db = Nat.mul k dc →
    IsRat (f a b) nc dc

Proof state:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂✝ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
h₂ : ↑nc * ↑da * ↑db * ⅟↑db * ⅟↑da * ⅟↑dc = ↑nc * ↑k * ↑dc * ⅟↑da * ⅟↑db * ⅟↑dc
⊢ ↑k * ↑nc * ⅟↑da * ⅟↑db = ↑nc * ⅟↑dc

Tactic:
simp only [mul_invOf_cancel_right'] at h₂

Result:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂✝ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
h₂ : ↑nc * ⅟↑dc = ↑nc * ↑k * ↑dc * ⅟↑da * ⅟↑db * ⅟↑dc
⊢ ↑k * ↑nc * ⅟↑da * ⅟↑db = ↑nc * ⅟↑dc

Proof state:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑(na * nb) = ↑(↑k * nc)
⊢ ↑na * ⅟↑da * (↑nb * ⅟↑db) = ↑nc * ⅟↑dc

Tactic:
simp only [Int.cast_mul, Int.cast_natCast] at h₁

Result:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
⊢ ↑na * ⅟↑da * (↑nb * ⅟↑db) = ↑nc * ⅟↑dc

Proof state:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂✝ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
h₂ : ↑nc * ⅟↑dc = ↑nc * ↑k * ↑dc * ⅟↑da * ⅟↑db * ⅟↑dc
⊢ ↑k * ↑nc * ⅟↑da * ⅟↑db = ↑nc * ⅟↑dc

Tactic:
rw [h₂, Nat.cast_commute]

Result:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂✝ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
h₂ : ↑nc * ⅟↑dc = ↑nc * ↑k * ↑dc * ⅟↑da * ⅟↑db * ⅟↑dc
⊢ ↑nc * ↑k * ⅟↑da * ⅟↑db = ↑nc * ↑k * ↑dc * ⅟↑da * ⅟↑db * ⅟↑dc

Proof state:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂✝ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
h₂ : (fun x => ↑nc * ↑x * (⅟↑da * ⅟↑db * ⅟↑dc)) (da * db) = (fun x => ↑nc * ↑x * (⅟↑da * ⅟↑db * ⅟↑dc)) (k * dc)
⊢ ↑k * ↑nc * ⅟↑da * ⅟↑db = ↑nc * ⅟↑dc

Tactic:
simp only [Nat.cast_mul, ← mul_assoc] at h₂

Result:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂✝ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
h₂ : ↑nc * ↑da * ↑db * ⅟↑da * ⅟↑db * ⅟↑dc = ↑nc * ↑k * ↑dc * ⅟↑da * ⅟↑db * ⅟↑dc
⊢ ↑k * ↑nc * ⅟↑da * ⅟↑db = ↑nc * ⅟↑dc

Proof state:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁ : na * nb = ↑k * nc
h₂ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
⊢ ↑na * ⅟↑da * (↑nb * ⅟↑db) = ↑nc * ⅟↑dc

Tactic:
have H := (Nat.cast_commute (α := α) da db).invOf_left.invOf_right.right_comm

Result:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁ : na * nb = ↑k * nc
h₂ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
⊢ ↑na * ⅟↑da * (↑nb * ⅟↑db) = ↑nc * ⅟↑dc

Proof state:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁ : na * nb = ↑k * nc
h₂ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
⊢ ↑na * ⅟↑da * (↑nb * ⅟↑db) = ↑nc * ⅟↑dc

Tactic:
have h₁ := congr_arg (Int.cast (R := α)) h₁

Result:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑(na * nb) = ↑(↑k * nc)
⊢ ↑na * ⅟↑da * (↑nb * ⅟↑db) = ↑nc * ⅟↑dc

Proof state:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁ : na * nb = ↑k * nc
h₂ : da * db = k * dc
this : Invertible ↑(da * db)
⊢ IsRat (↑na * ⅟↑da * (↑nb * ⅟↑db)) nc dc

Tactic:
have := invertibleOfMul' (α := α) h₂

Result:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁ : na * nb = ↑k * nc
h₂ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
⊢ IsRat (↑na * ⅟↑da * (↑nb * ⅟↑db)) nc dc

Proof state:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
⊢ ↑na * ⅟↑da * (↑nb * ⅟↑db) = ↑nc * ⅟↑dc

Tactic:
simp only [← mul_assoc, (Nat.cast_commute (α := α) da nb).invOf_left.right_comm, h₁]

Result:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
⊢ ↑k * ↑nc * ⅟↑da * ⅟↑db = ↑nc * ⅟↑dc

Proof state:
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁ : na * nb = ↑k * nc
h₂ : da * db = k * dc
⊢ Invertible ↑(da * db)

Tactic:
simpa using invertibleMul (da:α) db

Result:
no goals

Proof state:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂✝ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
h₂ : ↑nc * ↑da * ↑db * ⅟↑da * ⅟↑db * ⅟↑dc = ↑nc * ↑k * ↑dc * ⅟↑da * ⅟↑db * ⅟↑dc
⊢ ↑k * ↑nc * ⅟↑da * ⅟↑db = ↑nc * ⅟↑dc

Tactic:
rw [H] at h₂

Result:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁✝ : na * nb = ↑k * nc
h₂✝ : da * db = k * dc
this✝ : Invertible ↑(da * db)
this : Invertible ↑dc
H : ∀ (a : α), a * ⅟↑da * ⅟↑db = a * ⅟↑db * ⅟↑da
h₁ : ↑na * ↑nb = ↑k * ↑nc
h₂ : ↑nc * ↑da * ↑db * ⅟↑db * ⅟↑da * ⅟↑dc = ↑nc * ↑k * ↑dc * ⅟↑da * ⅟↑db * ⅟↑dc
⊢ ↑k * ↑nc * ⅟↑da * ⅟↑db = ↑nc * ⅟↑dc

Proof state:
α : Type u_1
inst✝ : Ring α
f : α → α → α
a b : α
na nb nc : ℤ
da db dc k : ℕ
⊢ f = HMul.hMul → IsRat a na da → IsRat b nb db → na.mul nb = (↑k).mul nc → da.mul db = k.mul dc → IsRat (f a b) nc dc

Tactic:
rintro rfl ⟨_, rfl⟩ ⟨_, rfl⟩ (h₁ : na * nb = k * nc) (h₂ : da * db = k * dc)

Result:
case mk.mk
α : Type u_1
inst✝ : Ring α
na nb nc : ℤ
da db dc k : ℕ
inv✝¹ : Invertible ↑da
inv✝ : Invertible ↑db
h₁ : na * nb = ↑k * nc
h₂ : da * db = k * dc
⊢ IsRat (↑na * ⅟↑da * (↑nb * ⅟↑db)) nc dc