Module: Mathlib.Geometry.Euclidean.Circumcenter

Goal state before:
pp: V : Type u_1
P : Type u_2
inst✝⁴ : NormedAddCommGroup V
inst✝³ : InnerProductSpace ℝ V
inst✝² : MetricSpace P
inst✝¹ : NormedAddTorsor V P
s : AffineSubspace ℝ P
inst✝ : s.direction.HasOrthogonalProjection
ps : Set P
hnps : ps.Nonempty
p : P
hps : ps ⊆ ↑s
hp : p ∉ s
hu : ∃! cs, cs.center ∈ s ∧ ps ⊆ Metric.sphere cs.center cs.radius
⊢ ∃! cs₂, cs₂.center ∈ affineSpan ℝ (insert p ↑s) ∧ insert p ps ⊆ Metric.sphere cs₂.center cs₂.radius

Tactic:
have : Nonempty s := Set.Nonempty.to_subtype (hnps.mono hps)
  rcases hu with ⟨⟨cc, cr⟩, ⟨hcc, hcr⟩, hcccru⟩
  simp only at hcc hcr hcccru
  let x := dist cc (orthogonalProjection s p)
  let y := dist p (orthogonalProjection s p)
  have hy0 : y ≠ 0 := dist_orthogonalProjection_ne_zero_of_notMem hp
  let ycc₂ := (x * x + y * y - cr * cr) / (2 * y)
  let cc₂ := (ycc₂ / y) • (p -ᵥ orthogonalProjection s p : V) +ᵥ cc
  let cr₂ := √(cr * cr + ycc₂ * ycc₂)
  use ⟨cc₂, cr₂⟩
  simp -zeta -proj only
  have hpo : p = (1 : ℝ) • (p -ᵥ orthogonalProjection s p : V) +ᵥ
    (orthogonalProjection s p : P) := by
    simp
  constructor
  · constructor
    · refine vadd_mem_of_mem_direction ?_ (mem_affineSpan ℝ (Set.mem_insert_of_mem _ hcc))
      rw [direction_affineSpan]
      exact
        Submodule.smul_mem _ _
          (vsub_mem_vectorSpan ℝ (Set.mem_insert _ _)
            (Set.mem_insert_of_mem _ (orthogonalProjection_mem _)))
    · intro p₁ hp₁
      rw [Sphere.mem_coe, mem_sphere, ← mul_self_inj_of_nonneg dist_nonneg (Real.sqrt_nonneg _),
        Real.mul_self_sqrt (add_nonneg (mul_self_nonneg _) (mul_self_nonneg _))]
      rcases hp₁ with hp₁ | hp₁
      · rw [hp₁, hpo,
          dist_sq_smul_orthogonal_vadd_smul_orthogonal_vadd (orthogonalProjection_mem p) hcc _ _
            (vsub_orthogonalProjection_mem_direction_orthogonal s p),
          Real.norm_eq_abs, abs_mul_abs_self, ← dist_eq_norm_vsub V p, dist_comm _ cc]
        simp only [ycc₂]
        field
      · rw [dist_sq_eq_dist_orthogonalProjection_sq_add_dist_orthogonalProjection_sq _ (hps hp₁),
          orthogonalProjection_vadd_smul_vsub_orthogonalProjection _ _ hcc, Subtype.coe_mk,
          dist_of_mem_subset_mk_sphere hp₁ hcr, dist_eq_norm_vsub V cc₂ cc, vadd_vsub, norm_smul, ←
          dist_eq_norm_vsub V, Real.norm_eq_abs, abs_div, abs_of_nonneg dist_nonneg,
          div_mul_cancel₀ _ hy0, abs_mul_abs_self]
  · rintro ⟨cc₃, cr₃⟩ ⟨hcc₃, hcr₃⟩
    simp only at hcc₃ hcr₃
    obtain ⟨t₃, cc₃', hcc₃', hcc₃''⟩ :
      ∃ r : ℝ, ∃ p0 ∈ s, cc₃ = r • (p -ᵥ ↑((orthogonalProjection s) p)) +ᵥ p0 := by
      rwa [mem_affineSpan_insert_iff (orthogonalProjection_mem p)] at hcc₃
    have hcr₃' : ∃ r, ∀ p₁ ∈ ps, dist p₁ cc₃ = r :=
      ⟨cr₃, fun p₁ hp₁ => dist_of_mem_subset_mk_sphere (Set.mem_insert_of_mem _ hp₁) hcr₃⟩
    rw [exists_dist_eq_iff_exists_dist_orthogonalProjection_eq hps cc₃, hcc₃'',
      orthogonalProjection_vadd_smul_vsub_orthogonalProjection _ _ hcc₃'] at hcr₃'
    obtain ⟨cr₃', hcr₃'⟩ := hcr₃'
    have hu := hcccru ⟨cc₃', cr₃'⟩
    simp only at hu
    replace hu := hu ⟨hcc₃', hcr₃'⟩
    cases hu
    have hcr₃val : cr₃ = √(cr * cr + t₃ * y * (t₃ * y)) := by
      obtain ⟨p0, hp0⟩ := hnps
      have h' : ↑(⟨cc, hcc₃'⟩ : s) = cc := rfl
      rw [← dist_of_mem_subset_mk_sphere (Set.mem_insert_of_mem _ hp0) hcr₃, hcc₃'', ←
        mul_self_inj_of_nonneg dist_nonneg (Real.sqrt_nonneg _),
        Real.mul_self_sqrt (add_nonneg (mul_self_nonneg _) (mul_self_nonneg _)),
        dist_sq_eq_dist_orthogonalProjection_sq_add_dist_orthogonalProjection_sq _ (hps hp0),
        orthogonalProjection_vadd_smul_vsub_orthogonalProjection _ _ hcc₃', h',
        dist_of_mem_subset_mk_sphere hp0 hcr, dist_eq_norm_vsub V _ cc, vadd_vsub, norm_smul, ←
        dist_eq_norm_vsub V p, Real.norm_eq_abs, ← mul_assoc, mul_comm _ |t₃|, ← mul_assoc,
        abs_mul_abs_self]
      ring
    replace hcr₃ := dist_of_mem_subset_mk_sphere (Set.mem_insert _ _) hcr₃
    rw [hpo, hcc₃'', hcr₃val, ← mul_self_inj_of_nonneg dist_nonneg (Real.sqrt_nonneg _),
      dist_sq_smul_orthogonal_vadd_smul_orthogonal_vadd (orthogonalProjection_mem p) hcc₃' _ _
        (vsub_orthogonalProjection_mem_direction_orthogonal s p),
      Real.norm_eq_abs, abs_mul_abs_self, dist_comm, ← dist_eq_norm_vsub V p,
      Real.mul_self_sqrt (add_nonneg (mul_self_nonneg _) (mul_self_nonneg _))] at hcr₃
    have : t₃ * y = ycc₂ := by grind
    grind

Goal state after: