-- Mathlib/MeasureTheory/Measure/Lebesgue/EqHaar.lean
theorem tendsto_addHaar_inter_smul_zero_of_density_zero_aux1 (s : Set E) (x : E)
    (h : Tendsto (fun r => μ (s ∩ closedBall x r) / μ (closedBall x r)) (𝓝[>] 0) (𝓝 0)) (t : Set E)
    (u : Set E) (h'u : μ u ≠ 0) (t_bound : t ⊆ closedBall 0 1) :
    Tendsto (fun r : ℝ => μ (s ∩ ({x} + r • t)) / μ ({x} + r • u)) (𝓝[>] 0) (𝓝 0)

Proof state:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
s : Set E
x : E
h : Tendsto (fun r => μ (s ∩ closedBall x r) / μ (closedBall x r)) (𝓝[>] 0) (𝓝 0)
t u : Set E
h'u : μ u ≠ 0
t_bound : t ⊆ closedBall 0 1
A : Tendsto (fun r => μ (s ∩ ({x} + r • t)) / μ (closedBall x r)) (𝓝[>] 0) (𝓝 0)
B : Tendsto (fun r => μ (closedBall x r) / μ ({x} + r • u)) (𝓝[>] 0) (𝓝 (μ (closedBall x 1) / μ ({x} + u)))
⊢ Tendsto (fun r => μ (s ∩ ({x} + r • t)) / μ ({x} + r • u)) (𝓝[>] 0) (𝓝 0)

Tactic:
have C : Tendsto (fun r : ℝ =>
      μ (s ∩ ({x} + r • t)) / μ (closedBall x r) * (μ (closedBall x r) / μ ({x} + r • u)))
    (𝓝[>] 0) (𝓝 (0 * (μ (closedBall x 1) / μ ({x} + u)))) := by
  apply ENNReal.Tendsto.mul A _ B (Or.inr ENNReal.zero_ne_top)
  simp [ENNReal.div_eq_top, h'u, measure_closedBall_lt_top.ne]

Result:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
s : Set E
x : E
h : Tendsto (fun r => μ (s ∩ closedBall x r) / μ (closedBall x r)) (𝓝[>] 0) (𝓝 0)
t u : Set E
h'u : μ u ≠ 0
t_bound : t ⊆ closedBall 0 1
A : Tendsto (fun r => μ (s ∩ ({x} + r • t)) / μ (closedBall x r)) (𝓝[>] 0) (𝓝 0)
B : Tendsto (fun r => μ (closedBall x r) / μ ({x} + r • u)) (𝓝[>] 0) (𝓝 (μ (closedBall x 1) / μ ({x} + u)))
C :
  Tendsto (fun r => μ (s ∩ ({x} + r • t)) / μ (closedBall x r) * (μ (closedBall x r) / μ ({x} + r • u))) (𝓝[>] 0)
    (𝓝 (0 * (μ (closedBall x 1) / μ ({x} + u))))
⊢ Tendsto (fun r => μ (s ∩ ({x} + r • t)) / μ ({x} + r • u)) (𝓝[>] 0) (𝓝 0)

Proof state:
case h
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
s : Set E
x : E
h : Tendsto (fun r => μ (s ∩ closedBall x r) / μ (closedBall x r)) (𝓝[>] 0) (𝓝 0)
t u : Set E
h'u : μ u ≠ 0
t_bound : t ⊆ closedBall 0 1
A : Tendsto (fun r => μ (s ∩ ({x} + r • t)) / μ (closedBall x r)) (𝓝[>] 0) (𝓝 0)
B : Tendsto (fun r => μ (closedBall x r) / μ ({x} + r • u)) (𝓝[>] 0) (𝓝 (μ (closedBall x 1) / μ ({x} + u)))
C :
  Tendsto (fun r => μ (s ∩ ({x} + r • t)) / μ (closedBall x r) * (μ (closedBall x r) / μ ({x} + r • u))) (𝓝[>] 0) (𝓝 0)
⊢ ∀ a ∈ Ioi 0,
    μ (s ∩ ({x} + a • t)) / μ (closedBall x a) * (μ (closedBall x a) / μ ({x} + a • u)) =
      μ (s ∩ ({x} + a • t)) / μ ({x} + a • u)

Tactic:
rintro r (rpos : 0 < r)

Result:
case h
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : MeasurableSpace E
inst✝² : BorelSpace E
inst✝¹ : FiniteDimensional ℝ E
μ : Measure E
inst✝ : μ.IsAddHaarMeasure
s : Set E
x : E
h : Tendsto (fun r => μ (s ∩ closedBall x r) / μ (closedBall x r)) (𝓝[>] 0) (𝓝 0)
t u : Set E
h'u : μ u ≠ 0
t_bound : t ⊆ closedBall 0 1
A : Tendsto (fun r => μ (s ∩ ({x} + r • t)) / μ (closedBall x r)) (𝓝[>] 0) (𝓝 0)
B : Tendsto (fun r => μ (closedBall x r) / μ ({x} + r • u)) (𝓝[>] 0) (𝓝 (μ (closedBall x 1) / μ ({x} + u)))
C :
  Tendsto (fun r => μ (s ∩ ({x} + r • t)) / μ (closedBall x r) * (μ (closedBall x r) / μ ({x} + r • u))) (𝓝[>] 0) (𝓝 0)
r : ℝ
rpos : 0 < r
⊢ μ (s ∩ ({x} + r • t)) / μ (closedBall x r) * (μ (closedBall x r) / μ ({x} + r • u)) =
    μ (s ∩ ({x} + r • t)) / μ ({x} + r • u)