-- Mathlib/Geometry/Euclidean/MongePoint.lean
theorem OrthocentricSystem.exists_circumradius_eq {s : Set P} (ho : OrthocentricSystem s) :
    ∃ r : ℝ, ∀ t : Triangle ℝ P, Set.range t.points ⊆ s → t.circumradius = r

Proof state:
case h.intro.intro
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
s : Set P
t : Triangle ℝ P
hto : t.orthocenter ∉ Set.range t.points
hts : s = insert t.orthocenter (Set.range t.points)
t₂ : Triangle ℝ P
ht₂ : Set.range t₂.points ⊆ insert t.orthocenter (Set.range t.points)
ht₂s : Set.range t₂.points ⊆ s
c : P
hc : c ∈ affineSpan ℝ (Set.range t.points)
h : ∀ (i : Fin 3), dist (t₂.points i) c = Simplex.circumradius t
⊢ Simplex.circumradius t₂ = Simplex.circumradius t

Tactic:
have hs : Set.range t.points ⊆ s := by
  rw [hts]
  exact Set.subset_insert _ _

Result:
case h.intro.intro
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
s : Set P
t : Triangle ℝ P
hto : t.orthocenter ∉ Set.range t.points
hts : s = insert t.orthocenter (Set.range t.points)
t₂ : Triangle ℝ P
ht₂ : Set.range t₂.points ⊆ insert t.orthocenter (Set.range t.points)
ht₂s : Set.range t₂.points ⊆ s
c : P
hc : c ∈ affineSpan ℝ (Set.range t.points)
h : ∀ (i : Fin 3), dist (t₂.points i) c = Simplex.circumradius t
hs : Set.range t.points ⊆ s
⊢ Simplex.circumradius t₂ = Simplex.circumradius t

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
s : Set P
ho : OrthocentricSystem s
⊢ ∃ r, ∀ (t : Triangle ℝ P), Set.range t.points ⊆ s → Simplex.circumradius t = r

Tactic:
rcases ho with ⟨t, hto, hts⟩

Result:
case intro.intro
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
s : Set P
t : Triangle ℝ P
hto : t.orthocenter ∉ Set.range t.points
hts : s = insert t.orthocenter (Set.range t.points)
⊢ ∃ r, ∀ (t : Triangle ℝ P), Set.range t.points ⊆ s → Simplex.circumradius t = r

Proof state:
case h
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
s : Set P
t : Triangle ℝ P
hto : t.orthocenter ∉ Set.range t.points
hts : s = insert t.orthocenter (Set.range t.points)
⊢ ∀ (t_1 : Triangle ℝ P), Set.range t_1.points ⊆ s → Simplex.circumradius t_1 = Simplex.circumradius t

Tactic:
intro t₂ ht₂

Result:
case h
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
s : Set P
t : Triangle ℝ P
hto : t.orthocenter ∉ Set.range t.points
hts : s = insert t.orthocenter (Set.range t.points)
t₂ : Triangle ℝ P
ht₂ : Set.range t₂.points ⊆ s
⊢ Simplex.circumradius t₂ = Simplex.circumradius t

Proof state:
case h
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
s : Set P
t : Triangle ℝ P
hto : t.orthocenter ∉ Set.range t.points
hts : s = insert t.orthocenter (Set.range t.points)
t₂ : Triangle ℝ P
ht₂ ht₂s : Set.range t₂.points ⊆ s
⊢ Simplex.circumradius t₂ = Simplex.circumradius t

Tactic:
rw [hts] at ht₂

Result:
case h
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
s : Set P
t : Triangle ℝ P
hto : t.orthocenter ∉ Set.range t.points
hts : s = insert t.orthocenter (Set.range t.points)
t₂ : Triangle ℝ P
ht₂ : Set.range t₂.points ⊆ insert t.orthocenter (Set.range t.points)
ht₂s : Set.range t₂.points ⊆ s
⊢ Simplex.circumradius t₂ = Simplex.circumradius t