theorem extracted_formal_statement_2.{u_1} {Ω : Type u_1} [inst : MeasurableSpace Ω] (μ : Measure Ω)
  [inst_1 : IsProbabilityMeasure μ] {n : ℕ} {X : Fin n → Ω → ℝ} (hXm : ∀ (i : Fin n), Measurable (X i))
  (hXi : iIndepFun X μ) (hX_bdd : ∀ (i : Fin n), ∀ᵐ (ω : Ω) ∂μ, |X i ω| ≤ 1)
  (hμX : ∀ (i : Fin n), ∫ (x : Ω), X i x ∂μ = 0) (σ : ℝ) (hσ : ∀ (i : Fin n), σ ^ 2 = ∫ (x : Ω), (X i ^ 2) x ∂μ)
  (ε τ : ℝ) (hτ : τ ≥ 0)
  (chernoff_bound :
    τ ≥ 0 →
      Integrable (fun ω => rexp (τ * ∑ i, X i ω)) μ →
        (μ {ω | ε ≤ ∑ i, X i ω}).toReal ≤ rexp (-τ * ε) * mgf (fun ω => ∑ i, X i ω) μ τ)
  (h_chernoff :
    Integrable (fun ω => rexp (τ * ∑ i, X i ω)) μ →
      (μ {ω | ε ≤ ∑ i, X i ω}).toReal ≤ rexp (-τ * ε) * mgf (fun ω => ∑ i, X i ω) μ τ)
  (h_integ : Integrable (fun ω => rexp (τ * ∑ i, X i ω)) μ)
  (h_ysum_mgf : ∀ (t : ℝ), mgf (fun ω => ∑ i, X i ω) μ t = ∏ i, mgf (X i) μ t)
  (h_original_goal : (μ {ω | ε ≤ ∑ i, X i ω}).toReal ≤ rexp (-(τ * ε) + ↑n * σ ^ 2 * (rexp τ - 1 - τ))) :
  rexp (-(τ * ε)) * ∏ x, ∫ (x_1 : Ω), rexp (τ * X x x_1) ∂μ ≤ rexp (-(τ * ε) + ↑n * σ ^ 2 * (rexp τ - 1 - τ)) := sorry

State before:
Ω : Type u_1
inst✝¹ : MeasurableSpace Ω
μ : Measure Ω
inst✝ : IsProbabilityMeasure μ
n : ℕ
X : Fin n → Ω → ℝ
hXm : ∀ (i : Fin n), Measurable (X i)
hXi : iIndepFun X μ
hX_bdd : ∀ (i : Fin n), ∀ᵐ (ω : Ω) ∂μ, |X i ω| ≤ 1
hμX : ∀ (i : Fin n), ∫ (x : Ω), X i x ∂μ = 0
σ : ℝ
hσ : ∀ (i : Fin n), σ ^ 2 = ∫ (x : Ω), (X i ^ 2) x ∂μ
ε τ : ℝ
hτ : τ ≥ 0
chernoff_bound :
  τ ≥ 0 →
    Integrable (fun ω => rexp (τ * ∑ i, X i ω)) μ →
      (μ {ω | ε ≤ ∑ i, X i ω}).toReal ≤ rexp (-τ * ε) * mgf (fun ω => ∑ i, X i ω) μ τ
h_chernoff :
  Integrable (fun ω => rexp (τ * ∑ i, X i ω)) μ →
    (μ {ω | ε ≤ ∑ i, X i ω}).toReal ≤ rexp (-τ * ε) * mgf (fun ω => ∑ i, X i ω) μ τ
h_integ : Integrable (fun ω => rexp (τ * ∑ i, X i ω)) μ
h_ysum_mgf : ∀ (t : ℝ), mgf (fun ω => ∑ i, X i ω) μ t = ∏ i, mgf (X i) μ t
⊢ (μ {ω | ε ≤ ∑ i, X i ω}).toReal ≤ rexp (-(τ * ε) + ↑n * σ ^ 2 * (rexp τ - 1 - τ))

Tactic: rw [h_ysum_mgf] at chernoff_bound
    simp [hτ, h_integ] at chernoff_bound
    suffices rexp (-(τ * ε)) * ∏ i, mgf (X i) μ τ ≤ rexp (-(τ * ε) + n * σ ^ 2 * (rexp τ - 1 - τ)) by linarith
    unfold mgf
    simp only

State after:
[Ω : Type u_1
 inst✝¹ : MeasurableSpace Ω
 μ : Measure Ω
 inst✝ : IsProbabilityMeasure μ
 n : ℕ
 X : Fin n → Ω → ℝ
 hXm : ∀ (i : Fin n), Measurable (X i)
 hXi : iIndepFun X μ
 hX_bdd : ∀ (i : Fin n), ∀ᵐ (ω : Ω) ∂μ, |X i ω| ≤ 1
 hμX : ∀ (i : Fin n), ∫ (x : Ω), X i x ∂μ = 0
 σ : ℝ
 hσ : ∀ (i : Fin n), σ ^ 2 = ∫ (x : Ω), (X i ^ 2) x ∂μ
 ε τ : ℝ
 hτ : τ ≥ 0
 h_chernoff :
   Integrable (fun ω => rexp (τ * ∑ i, X i ω)) μ →
     (μ {ω | ε ≤ ∑ i, X i ω}).toReal ≤ rexp (-τ * ε) * mgf (fun ω => ∑ i, X i ω) μ τ
 h_integ : Integrable (fun ω => rexp (τ * ∑ i, X i ω)) μ
 h_ysum_mgf : ∀ (t : ℝ), mgf (fun ω => ∑ i, X i ω) μ t = ∏ i, mgf (X i) μ t
 chernoff_bound : (μ {ω | ε ≤ ∑ i, X i ω}).toReal ≤ rexp (-(τ * ε)) * ∏ i, mgf (X i) μ τ
 ⊢ rexp (-(τ * ε)) * ∏ x, ∫ (x_1 : Ω), rexp (τ * X x x_1) ∂μ ≤ rexp (-(τ * ε) + ↑n * σ ^ 2 * (rexp τ - 1 - τ))]

Relevant lemmas: