-- Mathlib/Geometry/Euclidean/Circumcenter.lean
theorem reflection_circumcenter_eq_affineCombination_of_pointsWithCircumcenter {n : ℕ}
    (s : Simplex ℝ P n) {i₁ i₂ : Fin (n + 1)} (h : i₁ ≠ i₂) :
    reflection (affineSpan ℝ (s.points '' {i₁, i₂})) s.circumcenter =
      (univ : Finset (PointsWithCircumcenterIndex n)).affineCombination ℝ s.pointsWithCircumcenter
        (reflectionCircumcenterWeightsWithCircumcenter i₁ i₂)

Proof state:
V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
hc : #{i₁, i₂} = 2
W : AffineSubspace ℝ P := affineSpan ℝ (s.points '' {i₁, i₂})
⊢ (EuclideanGeometry.reflection (affineSpan ℝ (s.points '' {i₁, i₂}))) s.circumcenter =
    (affineCombination ℝ univ s.pointsWithCircumcenter) (reflectionCircumcenterWeightsWithCircumcenter i₁ i₂)

Tactic:
have h_faces :
  (orthogonalProjection W s.circumcenter : P) =
    ↑((s.face hc).orthogonalProjectionSpan s.circumcenter) := by
  apply eq_orthogonalProjection_of_eq_subspace
  simp [W]

Result:
V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
hc : #{i₁, i₂} = 2
W : AffineSubspace ℝ P := affineSpan ℝ (s.points '' {i₁, i₂})
h_faces :
  ↑((EuclideanGeometry.orthogonalProjection W) s.circumcenter) = ↑((s.face hc).orthogonalProjectionSpan s.circumcenter)
⊢ (EuclideanGeometry.reflection (affineSpan ℝ (s.points '' {i₁, i₂}))) s.circumcenter =
    (affineCombination ℝ univ s.pointsWithCircumcenter) (reflectionCircumcenterWeightsWithCircumcenter i₁ i₂)

Proof state:
V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
⊢ #{i₁, i₂} = 2

Tactic:
simp [h]

Result:
no goals

Proof state:
V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
⊢ (EuclideanGeometry.reflection (affineSpan ℝ (s.points '' {i₁, i₂}))) s.circumcenter =
    (affineCombination ℝ univ s.pointsWithCircumcenter) (reflectionCircumcenterWeightsWithCircumcenter i₁ i₂)

Tactic:
have hc : #{i₁, i₂} = 2 := by simp [h]

Result:
V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
hc : #{i₁, i₂} = 2
⊢ (EuclideanGeometry.reflection (affineSpan ℝ (s.points '' {i₁, i₂}))) s.circumcenter =
    (affineCombination ℝ univ s.pointsWithCircumcenter) (reflectionCircumcenterWeightsWithCircumcenter i₁ i₂)

Proof state:
V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
hc : #{i₁, i₂} = 2
W : AffineSubspace ℝ P := affineSpan ℝ (s.points '' {i₁, i₂})
h_faces :
  ↑((EuclideanGeometry.orthogonalProjection W) s.circumcenter) = ↑((s.face hc).orthogonalProjectionSpan s.circumcenter)
⊢ ∑ x : Fin (n + 1),
      ((if x ∈ {i₁, i₂} then ((↑2)⁻¹ + (↑2)⁻¹) • (s.pointsWithCircumcenter (pointIndex x) -ᵥ Classical.choice ⋯)
        else 0) -
        if x = i₁ ∨ x = i₂ then 1 • (s.pointsWithCircumcenter (pointIndex x) -ᵥ Classical.choice ⋯) else 0) =
    0

Tactic:
rw [← sum_const_zero]

Result:
V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
hc : #{i₁, i₂} = 2
W : AffineSubspace ℝ P := affineSpan ℝ (s.points '' {i₁, i₂})
h_faces :
  ↑((EuclideanGeometry.orthogonalProjection W) s.circumcenter) = ↑((s.face hc).orthogonalProjectionSpan s.circumcenter)
⊢ ∑ x : Fin (n + 1),
      ((if x ∈ {i₁, i₂} then ((↑2)⁻¹ + (↑2)⁻¹) • (s.pointsWithCircumcenter (pointIndex x) -ᵥ Classical.choice ⋯)
        else ∑ _x ∈ ?m.289042, 0) -
        if x = i₁ ∨ x = i₂ then 1 • (s.pointsWithCircumcenter (pointIndex x) -ᵥ Classical.choice ⋯)
        else ∑ _x ∈ ?m.289042, 0) =
    ∑ _x ∈ ?m.289042, 0

V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
hc : #{i₁, i₂} = 2
W : AffineSubspace ℝ P := affineSpan ℝ (s.points '' {i₁, i₂})
h_faces :
  ↑((EuclideanGeometry.orthogonalProjection W) s.circumcenter) = ↑((s.face hc).orthogonalProjectionSpan s.circumcenter)
⊢ Type ?u.289039

V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
hc : #{i₁, i₂} = 2
W : AffineSubspace ℝ P := affineSpan ℝ (s.points '' {i₁, i₂})
h_faces :
  ↑((EuclideanGeometry.orthogonalProjection W) s.circumcenter) = ↑((s.face hc).orthogonalProjectionSpan s.circumcenter)
⊢ Finset ?m.289040

Proof state:
V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
hc : #{i₁, i₂} = 2
⊢ (EuclideanGeometry.reflection (affineSpan ℝ (s.points '' {i₁, i₂}))) s.circumcenter =
    (affineCombination ℝ univ s.pointsWithCircumcenter) (reflectionCircumcenterWeightsWithCircumcenter i₁ i₂)

Tactic:
set W : AffineSubspace ℝ P := affineSpan ℝ (s.points '' {i₁, i₂})

Result:
V : Type u_1
P : Type u_2
inst✝³ : NormedAddCommGroup V
inst✝² : InnerProductSpace ℝ V
inst✝¹ : MetricSpace P
inst✝ : NormedAddTorsor V P
n : ℕ
s : Simplex ℝ P n
i₁ i₂ : Fin (n + 1)
h : i₁ ≠ i₂
hc : #{i₁, i₂} = 2
W : AffineSubspace ℝ P := affineSpan ℝ (s.points '' {i₁, i₂})
⊢ (EuclideanGeometry.reflection (affineSpan ℝ (s.points '' {i₁, i₂}))) s.circumcenter =
    (affineCombination ℝ univ s.pointsWithCircumcenter) (reflectionCircumcenterWeightsWithCircumcenter i₁ i₂)