l3lab/ntp-mathlib huggingface dataset:
theorem eta_fin_three (A : Matrix (Fin 3) (Fin 3) α) :
    A = !![A 0 0, A 0 1, A 0 2;
           A 1 0, A 1 1, A 1 2;
           A 2 0, A 2 1, A 2 2] := by
  ext i j
  fin_cases i <;> fin_cases j <;> 

Proof state:
case a.h.tail.head.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
A : Matrix (Fin 3) (Fin 3) α
⊢ A { val := 1, isLt := (_ : (fun a => a < 3) 1) } { val := 0, isLt := (_ : 0 < 3) } =
    of ![![A 0 0, A 0 1, A 0 2], ![A 1 0, A 1 1, A 1 2], ![A 2 0, A 2 1, A 2 2]]
      { val := 1, isLt := (_ : (fun a => a < 3) 1) } { val := 0, isLt := (_ : 0 < 3) }

Next tactic:
rfl
theorem eta_fin_three (A : Matrix (Fin 3) (Fin 3) α) :
    A = !![A 0 0, A 0 1, A 0 2;
           A 1 0, A 1 1, A 1 2;
           A 2 0, A 2 1, A 2 2] := by
  ext i j
  fin_cases i <;> fin_cases j <;> 

Proof state:
case a.h.tail.head.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
A : Matrix (Fin 3) (Fin 3) α
⊢ A { val := 1, isLt := (_ : (fun a => a < 3) 1) } { val := 1, isLt := (_ : (fun a => a < 3) 1) } =
    of ![![A 0 0, A 0 1, A 0 2], ![A 1 0, A 1 1, A 1 2], ![A 2 0, A 2 1, A 2 2]]
      { val := 1, isLt := (_ : (fun a => a < 3) 1) } { val := 1, isLt := (_ : (fun a => a < 3) 1) }

Next tactic:
rfl
theorem eta_fin_three (A : Matrix (Fin 3) (Fin 3) α) :
    A = !![A 0 0, A 0 1, A 0 2;
           A 1 0, A 1 1, A 1 2;
           A 2 0, A 2 1, A 2 2] := by
  ext i j
  fin_cases i <;> fin_cases j <;> 

Proof state:
case a.h.tail.head.tail.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
A : Matrix (Fin 3) (Fin 3) α
⊢ A { val := 1, isLt := (_ : (fun a => a < 3) 1) } { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) } =
    of ![![A 0 0, A 0 1, A 0 2], ![A 1 0, A 1 1, A 1 2], ![A 2 0, A 2 1, A 2 2]]
      { val := 1, isLt := (_ : (fun a => a < 3) 1) } { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) }

Next tactic:
rfl
theorem eta_fin_three (A : Matrix (Fin 3) (Fin 3) α) :
    A = !![A 0 0, A 0 1, A 0 2;
           A 1 0, A 1 1, A 1 2;
           A 2 0, A 2 1, A 2 2] := by
  ext i j
  fin_cases i <;> fin_cases j <;> 

Proof state:
case a.h.tail.tail.head.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
A : Matrix (Fin 3) (Fin 3) α
⊢ A { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) } { val := 0, isLt := (_ : 0 < 3) } =
    of ![![A 0 0, A 0 1, A 0 2], ![A 1 0, A 1 1, A 1 2], ![A 2 0, A 2 1, A 2 2]]
      { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) } { val := 0, isLt := (_ : 0 < 3) }

Next tactic:
rfl
theorem eta_fin_three (A : Matrix (Fin 3) (Fin 3) α) :
    A = !![A 0 0, A 0 1, A 0 2;
           A 1 0, A 1 1, A 1 2;
           A 2 0, A 2 1, A 2 2] := by
  ext i j
  fin_cases i <;> fin_cases j <;> 

Proof state:
case a.h.tail.tail.head.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
A : Matrix (Fin 3) (Fin 3) α
⊢ A { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) } { val := 1, isLt := (_ : (fun a => a < 3) 1) } =
    of ![![A 0 0, A 0 1, A 0 2], ![A 1 0, A 1 1, A 1 2], ![A 2 0, A 2 1, A 2 2]]
      { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) } { val := 1, isLt := (_ : (fun a => a < 3) 1) }

Next tactic:
rfl
theorem eta_fin_three (A : Matrix (Fin 3) (Fin 3) α) :
    A = !![A 0 0, A 0 1, A 0 2;
           A 1 0, A 1 1, A 1 2;
           A 2 0, A 2 1, A 2 2] := by
  ext i j
  fin_cases i <;> fin_cases j <;> 

Proof state:
case a.h.tail.tail.head.tail.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
A : Matrix (Fin 3) (Fin 3) α
⊢ A { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) }
      { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) } =
    of ![![A 0 0, A 0 1, A 0 2], ![A 1 0, A 1 1, A 1 2], ![A 2 0, A 2 1, A 2 2]]
      { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) }
      { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) }

Next tactic:
rfl
theorem mul_fin_two [AddCommMonoid α] [Mul α] (a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α) :
    !![a₁₁, a₁₂;
       a₂₁, a₂₂] * !![b₁₁, b₁₂;
                      b₂₁, b₂₂] = !![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂;
                                     a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂] := by
  

Proof state:
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α
⊢ of ![![a₁₁, a₁₂], ![a₂₁, a₂₂]] * of ![![b₁₁, b₁₂], ![b₂₁, b₂₂]] =
    of ![![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂], ![a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂]]

Next tactic:
ext i j
theorem mul_fin_two [AddCommMonoid α] [Mul α] (a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α) :
    !![a₁₁, a₁₂;
       a₂₁, a₂₂] * !![b₁₁, b₁₂;
                      b₂₁, b₂₂] = !![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂;
                                     a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂] := by
  ext i j
  

Proof state:
case a.h
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α
i j : Fin 2
⊢ (of ![![a₁₁, a₁₂], ![a₂₁, a₂₂]] * of ![![b₁₁, b₁₂], ![b₂₁, b₂₂]]) i j =
    of ![![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂], ![a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂]] i j

Next tactic:
fin_cases i
theorem mul_fin_two [AddCommMonoid α] [Mul α] (a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α) :
    !![a₁₁, a₁₂;
       a₂₁, a₂₂] * !![b₁₁, b₁₂;
                      b₂₁, b₂₂] = !![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂;
                                     a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂] := by
  ext i j
  fin_cases i <;> 

Proof state:
case a.h.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α
j : Fin 2
⊢ (of ![![a₁₁, a₁₂], ![a₂₁, a₂₂]] * of ![![b₁₁, b₁₂], ![b₂₁, b₂₂]]) { val := 0, isLt := (_ : 0 < 2) } j =
    of ![![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂], ![a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂]]
      { val := 0, isLt := (_ : 0 < 2) } j

Next tactic:
fin_cases j
theorem mul_fin_two [AddCommMonoid α] [Mul α] (a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α) :
    !![a₁₁, a₁₂;
       a₂₁, a₂₂] * !![b₁₁, b₁₂;
                      b₂₁, b₂₂] = !![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂;
                                     a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂] := by
  ext i j
  fin_cases i <;> 

Proof state:
case a.h.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α
j : Fin 2
⊢ (of ![![a₁₁, a₁₂], ![a₂₁, a₂₂]] * of ![![b₁₁, b₁₂], ![b₂₁, b₂₂]]) { val := 1, isLt := (_ : (fun a => a < 2) 1) } j =
    of ![![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂], ![a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂]]
      { val := 1, isLt := (_ : (fun a => a < 2) 1) } j

Next tactic:
fin_cases j
theorem mul_fin_two [AddCommMonoid α] [Mul α] (a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α) :
    !![a₁₁, a₁₂;
       a₂₁, a₂₂] * !![b₁₁, b₁₂;
                      b₂₁, b₂₂] = !![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂;
                                     a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂] := by
  ext i j
  fin_cases i <;> fin_cases j <;> 

Proof state:
case a.h.head.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α
⊢ (of ![![a₁₁, a₁₂], ![a₂₁, a₂₂]] * of ![![b₁₁, b₁₂], ![b₂₁, b₂₂]]) { val := 0, isLt := (_ : 0 < 2) }
      { val := 0, isLt := (_ : 0 < 2) } =
    of ![![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂], ![a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂]]
      { val := 0, isLt := (_ : 0 < 2) } { val := 0, isLt := (_ : 0 < 2) }

Next tactic:
simp [Matrix.mul_apply, dotProduct, Fin.sum_univ_succ]
theorem mul_fin_two [AddCommMonoid α] [Mul α] (a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α) :
    !![a₁₁, a₁₂;
       a₂₁, a₂₂] * !![b₁₁, b₁₂;
                      b₂₁, b₂₂] = !![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂;
                                     a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂] := by
  ext i j
  fin_cases i <;> fin_cases j <;> 

Proof state:
case a.h.head.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α
⊢ (of ![![a₁₁, a₁₂], ![a₂₁, a₂₂]] * of ![![b₁₁, b₁₂], ![b₂₁, b₂₂]]) { val := 0, isLt := (_ : 0 < 2) }
      { val := 1, isLt := (_ : (fun a => a < 2) 1) } =
    of ![![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂], ![a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂]]
      { val := 0, isLt := (_ : 0 < 2) } { val := 1, isLt := (_ : (fun a => a < 2) 1) }

Next tactic:
simp [Matrix.mul_apply, dotProduct, Fin.sum_univ_succ]
theorem mul_fin_two [AddCommMonoid α] [Mul α] (a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α) :
    !![a₁₁, a₁₂;
       a₂₁, a₂₂] * !![b₁₁, b₁₂;
                      b₂₁, b₂₂] = !![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂;
                                     a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂] := by
  ext i j
  fin_cases i <;> fin_cases j <;> 

Proof state:
case a.h.tail.head.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α
⊢ (of ![![a₁₁, a₁₂], ![a₂₁, a₂₂]] * of ![![b₁₁, b₁₂], ![b₂₁, b₂₂]]) { val := 1, isLt := (_ : (fun a => a < 2) 1) }
      { val := 0, isLt := (_ : 0 < 2) } =
    of ![![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂], ![a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂]]
      { val := 1, isLt := (_ : (fun a => a < 2) 1) } { val := 0, isLt := (_ : 0 < 2) }

Next tactic:
simp [Matrix.mul_apply, dotProduct, Fin.sum_univ_succ]
theorem mul_fin_two [AddCommMonoid α] [Mul α] (a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α) :
    !![a₁₁, a₁₂;
       a₂₁, a₂₂] * !![b₁₁, b₁₂;
                      b₂₁, b₂₂] = !![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂;
                                     a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂] := by
  ext i j
  fin_cases i <;> fin_cases j <;> 

Proof state:
case a.h.tail.head.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₂₁ a₂₂ b₁₁ b₁₂ b₂₁ b₂₂ : α
⊢ (of ![![a₁₁, a₁₂], ![a₂₁, a₂₂]] * of ![![b₁₁, b₁₂], ![b₂₁, b₂₂]]) { val := 1, isLt := (_ : (fun a => a < 2) 1) }
      { val := 1, isLt := (_ : (fun a => a < 2) 1) } =
    of ![![a₁₁ * b₁₁ + a₁₂ * b₂₁, a₁₁ * b₁₂ + a₁₂ * b₂₂], ![a₂₁ * b₁₁ + a₂₂ * b₂₁, a₂₁ * b₁₂ + a₂₂ * b₂₂]]
      { val := 1, isLt := (_ : (fun a => a < 2) 1) } { val := 1, isLt := (_ : (fun a => a < 2) 1) }

Next tactic:
simp [Matrix.mul_apply, dotProduct, Fin.sum_univ_succ]
theorem mul_fin_three [AddCommMonoid α] [Mul α]
    (a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α) :
    !![a₁₁, a₁₂, a₁₃;
       a₂₁, a₂₂, a₂₃;
       a₃₁, a₃₂, a₃₃] * !![b₁₁, b₁₂, b₁₃;
                           b₂₁, b₂₂, b₂₃;
                           b₃₁, b₃₂, b₃₃] =
    !![a₁₁*b₁₁ + a₁₂*b₂₁ + a₁₃*b₃₁, a₁₁*b₁₂ + a₁₂*b₂₂ + a₁₃*b₃₂, a₁₁*b₁₃ + a₁₂*b₂₃ + a₁₃*b₃₃;
       a₂₁*b₁₁ + a₂₂*b₂₁ + a₂₃*b₃₁, a₂₁*b₁₂ + a₂₂*b₂₂ + a₂₃*b₃₂, a₂₁*b₁₃ + a₂₂*b₂₃ + a₂₃*b₃₃;
       a₃₁*b₁₁ + a₃₂*b₂₁ + a₃₃*b₃₁, a₃₁*b₁₂ + a₃₂*b₂₂ + a₃₃*b₃₂, a₃₁*b₁₃ + a₃₂*b₂₃ + a₃₃*b₃₃] := by
  

Proof state:
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α
⊢ of ![![a₁₁, a₁₂, a₁₃], ![a₂₁, a₂₂, a₂₃], ![a₃₁, a₃₂, a₃₃]] *
      of ![![b₁₁, b₁₂, b₁₃], ![b₂₁, b₂₂, b₂₃], ![b₃₁, b₃₂, b₃₃]] =
    of
      ![![a₁₁ * b₁₁ + a₁₂ * b₂₁ + a₁₃ * b₃₁, a₁₁ * b₁₂ + a₁₂ * b₂₂ + a₁₃ * b₃₂, a₁₁ * b₁₃ + a₁₂ * b₂₃ + a₁₃ * b₃₃],
        ![a₂₁ * b₁₁ + a₂₂ * b₂₁ + a₂₃ * b₃₁, a₂₁ * b₁₂ + a₂₂ * b₂₂ + a₂₃ * b₃₂, a₂₁ * b₁₃ + a₂₂ * b₂₃ + a₂₃ * b₃₃],
        ![a₃₁ * b₁₁ + a₃₂ * b₂₁ + a₃₃ * b₃₁, a₃₁ * b₁₂ + a₃₂ * b₂₂ + a₃₃ * b₃₂, a₃₁ * b₁₃ + a₃₂ * b₂₃ + a₃₃ * b₃₃]]

Next tactic:
ext i j
theorem mul_fin_three [AddCommMonoid α] [Mul α]
    (a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α) :
    !![a₁₁, a₁₂, a₁₃;
       a₂₁, a₂₂, a₂₃;
       a₃₁, a₃₂, a₃₃] * !![b₁₁, b₁₂, b₁₃;
                           b₂₁, b₂₂, b₂₃;
                           b₃₁, b₃₂, b₃₃] =
    !![a₁₁*b₁₁ + a₁₂*b₂₁ + a₁₃*b₃₁, a₁₁*b₁₂ + a₁₂*b₂₂ + a₁₃*b₃₂, a₁₁*b₁₃ + a₁₂*b₂₃ + a₁₃*b₃₃;
       a₂₁*b₁₁ + a₂₂*b₂₁ + a₂₃*b₃₁, a₂₁*b₁₂ + a₂₂*b₂₂ + a₂₃*b₃₂, a₂₁*b₁₃ + a₂₂*b₂₃ + a₂₃*b₃₃;
       a₃₁*b₁₁ + a₃₂*b₂₁ + a₃₃*b₃₁, a₃₁*b₁₂ + a₃₂*b₂₂ + a₃₃*b₃₂, a₃₁*b₁₃ + a₃₂*b₂₃ + a₃₃*b₃₃] := by
  ext i j
  

Proof state:
case a.h
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α
i j : Fin 3
⊢ (of ![![a₁₁, a₁₂, a₁₃], ![a₂₁, a₂₂, a₂₃], ![a₃₁, a₃₂, a₃₃]] *
        of ![![b₁₁, b₁₂, b₁₃], ![b₂₁, b₂₂, b₂₃], ![b₃₁, b₃₂, b₃₃]])
      i j =
    of
      ![![a₁₁ * b₁₁ + a₁₂ * b₂₁ + a₁₃ * b₃₁, a₁₁ * b₁₂ + a₁₂ * b₂₂ + a₁₃ * b₃₂, a₁₁ * b₁₃ + a₁₂ * b₂₃ + a₁₃ * b₃₃],
        ![a₂₁ * b₁₁ + a₂₂ * b₂₁ + a₂₃ * b₃₁, a₂₁ * b₁₂ + a₂₂ * b₂₂ + a₂₃ * b₃₂, a₂₁ * b₁₃ + a₂₂ * b₂₃ + a₂₃ * b₃₃],
        ![a₃₁ * b₁₁ + a₃₂ * b₂₁ + a₃₃ * b₃₁, a₃₁ * b₁₂ + a₃₂ * b₂₂ + a₃₃ * b₃₂, a₃₁ * b₁₃ + a₃₂ * b₂₃ + a₃₃ * b₃₃]]
      i j

Next tactic:
fin_cases i
theorem mul_fin_three [AddCommMonoid α] [Mul α]
    (a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α) :
    !![a₁₁, a₁₂, a₁₃;
       a₂₁, a₂₂, a₂₃;
       a₃₁, a₃₂, a₃₃] * !![b₁₁, b₁₂, b₁₃;
                           b₂₁, b₂₂, b₂₃;
                           b₃₁, b₃₂, b₃₃] =
    !![a₁₁*b₁₁ + a₁₂*b₂₁ + a₁₃*b₃₁, a₁₁*b₁₂ + a₁₂*b₂₂ + a₁₃*b₃₂, a₁₁*b₁₃ + a₁₂*b₂₃ + a₁₃*b₃₃;
       a₂₁*b₁₁ + a₂₂*b₂₁ + a₂₃*b₃₁, a₂₁*b₁₂ + a₂₂*b₂₂ + a₂₃*b₃₂, a₂₁*b₁₃ + a₂₂*b₂₃ + a₂₃*b₃₃;
       a₃₁*b₁₁ + a₃₂*b₂₁ + a₃₃*b₃₁, a₃₁*b₁₂ + a₃₂*b₂₂ + a₃₃*b₃₂, a₃₁*b₁₃ + a₃₂*b₂₃ + a₃₃*b₃₃] := by
  ext i j
  fin_cases i <;> 

Proof state:
case a.h.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α
j : Fin 3
⊢ (of ![![a₁₁, a₁₂, a₁₃], ![a₂₁, a₂₂, a₂₃], ![a₃₁, a₃₂, a₃₃]] *
        of ![![b₁₁, b₁₂, b₁₃], ![b₂₁, b₂₂, b₂₃], ![b₃₁, b₃₂, b₃₃]])
      { val := 0, isLt := (_ : 0 < 3) } j =
    of
      ![![a₁₁ * b₁₁ + a₁₂ * b₂₁ + a₁₃ * b₃₁, a₁₁ * b₁₂ + a₁₂ * b₂₂ + a₁₃ * b₃₂, a₁₁ * b₁₃ + a₁₂ * b₂₃ + a₁₃ * b₃₃],
        ![a₂₁ * b₁₁ + a₂₂ * b₂₁ + a₂₃ * b₃₁, a₂₁ * b₁₂ + a₂₂ * b₂₂ + a₂₃ * b₃₂, a₂₁ * b₁₃ + a₂₂ * b₂₃ + a₂₃ * b₃₃],
        ![a₃₁ * b₁₁ + a₃₂ * b₂₁ + a₃₃ * b₃₁, a₃₁ * b₁₂ + a₃₂ * b₂₂ + a₃₃ * b₃₂, a₃₁ * b₁₃ + a₃₂ * b₂₃ + a₃₃ * b₃₃]]
      { val := 0, isLt := (_ : 0 < 3) } j

Next tactic:
fin_cases j
theorem mul_fin_three [AddCommMonoid α] [Mul α]
    (a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α) :
    !![a₁₁, a₁₂, a₁₃;
       a₂₁, a₂₂, a₂₃;
       a₃₁, a₃₂, a₃₃] * !![b₁₁, b₁₂, b₁₃;
                           b₂₁, b₂₂, b₂₃;
                           b₃₁, b₃₂, b₃₃] =
    !![a₁₁*b₁₁ + a₁₂*b₂₁ + a₁₃*b₃₁, a₁₁*b₁₂ + a₁₂*b₂₂ + a₁₃*b₃₂, a₁₁*b₁₃ + a₁₂*b₂₃ + a₁₃*b₃₃;
       a₂₁*b₁₁ + a₂₂*b₂₁ + a₂₃*b₃₁, a₂₁*b₁₂ + a₂₂*b₂₂ + a₂₃*b₃₂, a₂₁*b₁₃ + a₂₂*b₂₃ + a₂₃*b₃₃;
       a₃₁*b₁₁ + a₃₂*b₂₁ + a₃₃*b₃₁, a₃₁*b₁₂ + a₃₂*b₂₂ + a₃₃*b₃₂, a₃₁*b₁₃ + a₃₂*b₂₃ + a₃₃*b₃₃] := by
  ext i j
  fin_cases i <;> 

Proof state:
case a.h.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α
j : Fin 3
⊢ (of ![![a₁₁, a₁₂, a₁₃], ![a₂₁, a₂₂, a₂₃], ![a₃₁, a₃₂, a₃₃]] *
        of ![![b₁₁, b₁₂, b₁₃], ![b₂₁, b₂₂, b₂₃], ![b₃₁, b₃₂, b₃₃]])
      { val := 1, isLt := (_ : (fun a => a < 3) 1) } j =
    of
      ![![a₁₁ * b₁₁ + a₁₂ * b₂₁ + a₁₃ * b₃₁, a₁₁ * b₁₂ + a₁₂ * b₂₂ + a₁₃ * b₃₂, a₁₁ * b₁₃ + a₁₂ * b₂₃ + a₁₃ * b₃₃],
        ![a₂₁ * b₁₁ + a₂₂ * b₂₁ + a₂₃ * b₃₁, a₂₁ * b₁₂ + a₂₂ * b₂₂ + a₂₃ * b₃₂, a₂₁ * b₁₃ + a₂₂ * b₂₃ + a₂₃ * b₃₃],
        ![a₃₁ * b₁₁ + a₃₂ * b₂₁ + a₃₃ * b₃₁, a₃₁ * b₁₂ + a₃₂ * b₂₂ + a₃₃ * b₃₂, a₃₁ * b₁₃ + a₃₂ * b₂₃ + a₃₃ * b₃₃]]
      { val := 1, isLt := (_ : (fun a => a < 3) 1) } j

Next tactic:
fin_cases j
theorem mul_fin_three [AddCommMonoid α] [Mul α]
    (a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α) :
    !![a₁₁, a₁₂, a₁₃;
       a₂₁, a₂₂, a₂₃;
       a₃₁, a₃₂, a₃₃] * !![b₁₁, b₁₂, b₁₃;
                           b₂₁, b₂₂, b₂₃;
                           b₃₁, b₃₂, b₃₃] =
    !![a₁₁*b₁₁ + a₁₂*b₂₁ + a₁₃*b₃₁, a₁₁*b₁₂ + a₁₂*b₂₂ + a₁₃*b₃₂, a₁₁*b₁₃ + a₁₂*b₂₃ + a₁₃*b₃₃;
       a₂₁*b₁₁ + a₂₂*b₂₁ + a₂₃*b₃₁, a₂₁*b₁₂ + a₂₂*b₂₂ + a₂₃*b₃₂, a₂₁*b₁₃ + a₂₂*b₂₃ + a₂₃*b₃₃;
       a₃₁*b₁₁ + a₃₂*b₂₁ + a₃₃*b₃₁, a₃₁*b₁₂ + a₃₂*b₂₂ + a₃₃*b₃₂, a₃₁*b₁₃ + a₃₂*b₂₃ + a₃₃*b₃₃] := by
  ext i j
  fin_cases i <;> 

Proof state:
case a.h.tail.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α
j : Fin 3
⊢ (of ![![a₁₁, a₁₂, a₁₃], ![a₂₁, a₂₂, a₂₃], ![a₃₁, a₃₂, a₃₃]] *
        of ![![b₁₁, b₁₂, b₁₃], ![b₂₁, b₂₂, b₂₃], ![b₃₁, b₃₂, b₃₃]])
      { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) } j =
    of
      ![![a₁₁ * b₁₁ + a₁₂ * b₂₁ + a₁₃ * b₃₁, a₁₁ * b₁₂ + a₁₂ * b₂₂ + a₁₃ * b₃₂, a₁₁ * b₁₃ + a₁₂ * b₂₃ + a₁₃ * b₃₃],
        ![a₂₁ * b₁₁ + a₂₂ * b₂₁ + a₂₃ * b₃₁, a₂₁ * b₁₂ + a₂₂ * b₂₂ + a₂₃ * b₃₂, a₂₁ * b₁₃ + a₂₂ * b₂₃ + a₂₃ * b₃₃],
        ![a₃₁ * b₁₁ + a₃₂ * b₂₁ + a₃₃ * b₃₁, a₃₁ * b₁₂ + a₃₂ * b₂₂ + a₃₃ * b₃₂, a₃₁ * b₁₃ + a₃₂ * b₂₃ + a₃₃ * b₃₃]]
      { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) } j

Next tactic:
fin_cases j
theorem mul_fin_three [AddCommMonoid α] [Mul α]
    (a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α) :
    !![a₁₁, a₁₂, a₁₃;
       a₂₁, a₂₂, a₂₃;
       a₃₁, a₃₂, a₃₃] * !![b₁₁, b₁₂, b₁₃;
                           b₂₁, b₂₂, b₂₃;
                           b₃₁, b₃₂, b₃₃] =
    !![a₁₁*b₁₁ + a₁₂*b₂₁ + a₁₃*b₃₁, a₁₁*b₁₂ + a₁₂*b₂₂ + a₁₃*b₃₂, a₁₁*b₁₃ + a₁₂*b₂₃ + a₁₃*b₃₃;
       a₂₁*b₁₁ + a₂₂*b₂₁ + a₂₃*b₃₁, a₂₁*b₁₂ + a₂₂*b₂₂ + a₂₃*b₃₂, a₂₁*b₁₃ + a₂₂*b₂₃ + a₂₃*b₃₃;
       a₃₁*b₁₁ + a₃₂*b₂₁ + a₃₃*b₃₁, a₃₁*b₁₂ + a₃₂*b₂₂ + a₃₃*b₃₂, a₃₁*b₁₃ + a₃₂*b₂₃ + a₃₃*b₃₃] := by
  ext i j
  fin_cases i <;> fin_cases j
    <;> 

Proof state:
case a.h.head.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α
⊢ (of ![![a₁₁, a₁₂, a₁₃], ![a₂₁, a₂₂, a₂₃], ![a₃₁, a₃₂, a₃₃]] *
        of ![![b₁₁, b₁₂, b₁₃], ![b₂₁, b₂₂, b₂₃], ![b₃₁, b₃₂, b₃₃]])
      { val := 0, isLt := (_ : 0 < 3) } { val := 0, isLt := (_ : 0 < 3) } =
    of
      ![![a₁₁ * b₁₁ + a₁₂ * b₂₁ + a₁₃ * b₃₁, a₁₁ * b₁₂ + a₁₂ * b₂₂ + a₁₃ * b₃₂, a₁₁ * b₁₃ + a₁₂ * b₂₃ + a₁₃ * b₃₃],
        ![a₂₁ * b₁₁ + a₂₂ * b₂₁ + a₂₃ * b₃₁, a₂₁ * b₁₂ + a₂₂ * b₂₂ + a₂₃ * b₃₂, a₂₁ * b₁₃ + a₂₂ * b₂₃ + a₂₃ * b₃₃],
        ![a₃₁ * b₁₁ + a₃₂ * b₂₁ + a₃₃ * b₃₁, a₃₁ * b₁₂ + a₃₂ * b₂₂ + a₃₃ * b₃₂, a₃₁ * b₁₃ + a₃₂ * b₂₃ + a₃₃ * b₃₃]]
      { val := 0, isLt := (_ : 0 < 3) } { val := 0, isLt := (_ : 0 < 3) }

Next tactic:
simp [Matrix.mul_apply, dotProduct, Fin.sum_univ_succ, ← add_assoc]
theorem mul_fin_three [AddCommMonoid α] [Mul α]
    (a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α) :
    !![a₁₁, a₁₂, a₁₃;
       a₂₁, a₂₂, a₂₃;
       a₃₁, a₃₂, a₃₃] * !![b₁₁, b₁₂, b₁₃;
                           b₂₁, b₂₂, b₂₃;
                           b₃₁, b₃₂, b₃₃] =
    !![a₁₁*b₁₁ + a₁₂*b₂₁ + a₁₃*b₃₁, a₁₁*b₁₂ + a₁₂*b₂₂ + a₁₃*b₃₂, a₁₁*b₁₃ + a₁₂*b₂₃ + a₁₃*b₃₃;
       a₂₁*b₁₁ + a₂₂*b₂₁ + a₂₃*b₃₁, a₂₁*b₁₂ + a₂₂*b₂₂ + a₂₃*b₃₂, a₂₁*b₁₃ + a₂₂*b₂₃ + a₂₃*b₃₃;
       a₃₁*b₁₁ + a₃₂*b₂₁ + a₃₃*b₃₁, a₃₁*b₁₂ + a₃₂*b₂₂ + a₃₃*b₃₂, a₃₁*b₁₃ + a₃₂*b₂₃ + a₃₃*b₃₃] := by
  ext i j
  fin_cases i <;> fin_cases j
    <;> 

Proof state:
case a.h.head.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α
⊢ (of ![![a₁₁, a₁₂, a₁₃], ![a₂₁, a₂₂, a₂₃], ![a₃₁, a₃₂, a₃₃]] *
        of ![![b₁₁, b₁₂, b₁₃], ![b₂₁, b₂₂, b₂₃], ![b₃₁, b₃₂, b₃₃]])
      { val := 0, isLt := (_ : 0 < 3) } { val := 1, isLt := (_ : (fun a => a < 3) 1) } =
    of
      ![![a₁₁ * b₁₁ + a₁₂ * b₂₁ + a₁₃ * b₃₁, a₁₁ * b₁₂ + a₁₂ * b₂₂ + a₁₃ * b₃₂, a₁₁ * b₁₃ + a₁₂ * b₂₃ + a₁₃ * b₃₃],
        ![a₂₁ * b₁₁ + a₂₂ * b₂₁ + a₂₃ * b₃₁, a₂₁ * b₁₂ + a₂₂ * b₂₂ + a₂₃ * b₃₂, a₂₁ * b₁₃ + a₂₂ * b₂₃ + a₂₃ * b₃₃],
        ![a₃₁ * b₁₁ + a₃₂ * b₂₁ + a₃₃ * b₃₁, a₃₁ * b₁₂ + a₃₂ * b₂₂ + a₃₃ * b₃₂, a₃₁ * b₁₃ + a₃₂ * b₂₃ + a₃₃ * b₃₃]]
      { val := 0, isLt := (_ : 0 < 3) } { val := 1, isLt := (_ : (fun a => a < 3) 1) }

Next tactic:
simp [Matrix.mul_apply, dotProduct, Fin.sum_univ_succ, ← add_assoc]
theorem mul_fin_three [AddCommMonoid α] [Mul α]
    (a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α) :
    !![a₁₁, a₁₂, a₁₃;
       a₂₁, a₂₂, a₂₃;
       a₃₁, a₃₂, a₃₃] * !![b₁₁, b₁₂, b₁₃;
                           b₂₁, b₂₂, b₂₃;
                           b₃₁, b₃₂, b₃₃] =
    !![a₁₁*b₁₁ + a₁₂*b₂₁ + a₁₃*b₃₁, a₁₁*b₁₂ + a₁₂*b₂₂ + a₁₃*b₃₂, a₁₁*b₁₃ + a₁₂*b₂₃ + a₁₃*b₃₃;
       a₂₁*b₁₁ + a₂₂*b₂₁ + a₂₃*b₃₁, a₂₁*b₁₂ + a₂₂*b₂₂ + a₂₃*b₃₂, a₂₁*b₁₃ + a₂₂*b₂₃ + a₂₃*b₃₃;
       a₃₁*b₁₁ + a₃₂*b₂₁ + a₃₃*b₃₁, a₃₁*b₁₂ + a₃₂*b₂₂ + a₃₃*b₃₂, a₃₁*b₁₃ + a₃₂*b₂₃ + a₃₃*b₃₃] := by
  ext i j
  fin_cases i <;> fin_cases j
    <;> 

Proof state:
case a.h.head.tail.tail.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α
⊢ (of ![![a₁₁, a₁₂, a₁₃], ![a₂₁, a₂₂, a₂₃], ![a₃₁, a₃₂, a₃₃]] *
        of ![![b₁₁, b₁₂, b₁₃], ![b₂₁, b₂₂, b₂₃], ![b₃₁, b₃₂, b₃₃]])
      { val := 0, isLt := (_ : 0 < 3) } { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) } =
    of
      ![![a₁₁ * b₁₁ + a₁₂ * b₂₁ + a₁₃ * b₃₁, a₁₁ * b₁₂ + a₁₂ * b₂₂ + a₁₃ * b₃₂, a₁₁ * b₁₃ + a₁₂ * b₂₃ + a₁₃ * b₃₃],
        ![a₂₁ * b₁₁ + a₂₂ * b₂₁ + a₂₃ * b₃₁, a₂₁ * b₁₂ + a₂₂ * b₂₂ + a₂₃ * b₃₂, a₂₁ * b₁₃ + a₂₂ * b₂₃ + a₂₃ * b₃₃],
        ![a₃₁ * b₁₁ + a₃₂ * b₂₁ + a₃₃ * b₃₁, a₃₁ * b₁₂ + a₃₂ * b₂₂ + a₃₃ * b₃₂, a₃₁ * b₁₃ + a₃₂ * b₂₃ + a₃₃ * b₃₃]]
      { val := 0, isLt := (_ : 0 < 3) } { val := 2, isLt := (_ : (fun a => (fun a => a < 3) a) 2) }

Next tactic:
simp [Matrix.mul_apply, dotProduct, Fin.sum_univ_succ, ← add_assoc]
theorem mul_fin_three [AddCommMonoid α] [Mul α]
    (a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α) :
    !![a₁₁, a₁₂, a₁₃;
       a₂₁, a₂₂, a₂₃;
       a₃₁, a₃₂, a₃₃] * !![b₁₁, b₁₂, b₁₃;
                           b₂₁, b₂₂, b₂₃;
                           b₃₁, b₃₂, b₃₃] =
    !![a₁₁*b₁₁ + a₁₂*b₂₁ + a₁₃*b₃₁, a₁₁*b₁₂ + a₁₂*b₂₂ + a₁₃*b₃₂, a₁₁*b₁₃ + a₁₂*b₂₃ + a₁₃*b₃₃;
       a₂₁*b₁₁ + a₂₂*b₂₁ + a₂₃*b₃₁, a₂₁*b₁₂ + a₂₂*b₂₂ + a₂₃*b₃₂, a₂₁*b₁₃ + a₂₂*b₂₃ + a₂₃*b₃₃;
       a₃₁*b₁₁ + a₃₂*b₂₁ + a₃₃*b₃₁, a₃₁*b₁₂ + a₃₂*b₂₂ + a₃₃*b₃₂, a₃₁*b₁₃ + a₃₂*b₂₃ + a₃₃*b₃₃] := by
  ext i j
  fin_cases i <;> fin_cases j
    <;> 

Proof state:
case a.h.tail.head.head
α : Type u
o n m : ℕ
m' : Type uₘ
n' : Type uₙ
o' : Type uₒ
a b : ℕ
inst✝¹ : AddCommMonoid α
inst✝ : Mul α
a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ b₁₁ b₁₂ b₁₃ b₂₁ b₂₂ b₂₃ b₃₁ b₃₂ b₃₃ : α
⊢ (of ![![a₁₁, a₁₂, a₁₃], ![a₂₁, a₂₂, a₂₃], ![a₃₁, a₃₂, a₃₃]] *
        of ![![b₁₁, b₁₂, b₁₃], ![b₂₁, b₂₂, b₂₃], ![b₃₁, b₃₂, b₃₃]])
      { val := 1, isLt := (_ : (fun a => a < 3) 1) } { val := 0, isLt := (_ : 0 < 3) } =
    of
      ![![a₁₁ * b₁₁ + a₁₂ * b₂₁ + a₁₃ * b₃₁, a₁₁ * b₁₂ + a₁₂ * b₂₂ + a₁₃ * b₃₂, a₁₁ * b₁₃ + a₁₂ * b₂₃ + a₁₃ * b₃₃],
        ![a₂₁ * b₁₁ + a₂₂ * b₂₁ + a₂₃ * b₃₁, a₂₁ * b₁₂ + a₂₂ * b₂₂ + a₂₃ * b₃₂, a₂₁ * b₁₃ + a₂₂ * b₂₃ + a₂₃ * b₃₃],
        ![a₃₁ * b₁₁ + a₃₂ * b₂₁ + a₃₃ * b₃₁, a₃₁ * b₁₂ + a₃₂ * b₂₂ + a₃₃ * b₃₂, a₃₁ * b₁₃ + a₃₂ * b₂₃ + a₃₃ * b₃₃]]
      { val := 1, isLt := (_ : (fun a => a < 3) 1) } { val := 0, isLt := (_ : 0 < 3) }

Next tactic:
simp [Matrix.mul_apply, dotProduct, Fin.sum_univ_succ, ← add_assoc]