-- Lean 4 proof step from Mathlib.NumberTheory.ZetaValues (goal)
Goal:
k : ℕ
hk : k ≠ 0
x : ℝ
hx : Membership.mem (γ := Set ℝ) (Icc 0 1) x
n : ℕ
h :
  1 / HPow.hPow (α := ℂ) (↑n : ℂ) (2 * k + 1) *
      HSub.hSub (α := ℂ) (β := ℂ) ((fourier (↑n : ℤ) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ)
        ((fourier (-(↑n : ℤ)) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ) =
    1 / HPow.hPow (α := ℂ) (↑n : ℂ) (2 * k + 1) *
      HAdd.hAdd (α := ℂ) ((fourier (↑n : ℤ) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ)
        (HMul.hMul (β := ℂ) (HPow.hPow (α := ℂ) (-1) (2 * k) * HPow.hPow (α := ℂ) (β := ℕ) (-1) 1)
          ((fourier (-(↑n : ℤ)) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ))
h₁ :
  1 / HPow.hPow (α := ℂ) (↑n : ℂ) (2 * k + 1) *
      HSub.hSub (α := ℂ) (β := ℂ) ((fourier (↑n : ℤ) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ)
        ((fourier (-(↑n : ℤ)) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ) =
    1 / HPow.hPow (α := ℂ) (↑n : ℂ) (2 * k + 1) *
      HAdd.hAdd (α := ℂ) ((fourier (↑n : ℤ) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ)
        (HMul.hMul (β := ℂ) (HPow.hPow (α := ℂ) (β := ℕ) (-1) 2 ^ k * HPow.hPow (α := ℂ) (β := ℕ) (-1) 1)
          ((fourier (-(↑n : ℤ)) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ))
h₂ :
  1 / HPow.hPow (α := ℂ) (↑n : ℂ) (2 * k + 1) *
      HSub.hSub (α := ℂ) (β := ℂ) ((fourier (↑n : ℤ) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ)
        ((fourier (-(↑n : ℤ)) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ) =
    1 / HPow.hPow (α := ℂ) (↑n : ℂ) (2 * k + 1) *
      HAdd.hAdd (α := ℂ) ((fourier (↑n : ℤ) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ)
        (HMul.hMul (β := ℂ) (HPow.hPow (α := ℂ) 1 k * HPow.hPow (α := ℂ) (β := ℕ) (-1) 1)
          ((fourier (-(↑n : ℤ)) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ))
⊢ 1 / HPow.hPow (α := ℂ) (↑n : ℂ) (2 * k + 1) *
      HSub.hSub (α := ℂ) (β := ℂ) ((fourier (↑n : ℤ) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ)
        ((fourier (-(↑n : ℤ)) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ) =
    1 / HPow.hPow (α := ℂ) (↑n : ℂ) (2 * k + 1) *
      HAdd.hAdd (α := ℂ) ((fourier (↑n : ℤ) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ)
        (HMul.hMul (β := ℂ) (HPow.hPow (α := ℂ) (-1) (2 * k + 1))
          ((fourier (-(↑n : ℤ)) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ))
have h₃ : 1 / HPow.hPow (α := ℂ) (↑n : ℂ) (2 * k + 1) *
    HSub.hSub (α := ℂ) (β := ℂ) ((fourier (↑n : ℤ) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ)
      ((fourier (-(↑n : ℤ)) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ) =
  1 / HPow.hPow (α := ℂ) (↑n : ℂ) (2 * k + 1) *
    HAdd.hAdd (α := ℂ) ((fourier (↑n : ℤ) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ)
      (HMul.hMul (β := ℂ) (1 * HPow.hPow (α := ℂ) (β := ℕ) (-1) 1)
        ((fourier (-(↑n : ℤ)) : (a : AddCircle 1) → ℂ) (↑x : ℝ ⧸ AddSubgroup.zmultiples 1) : ℂ))
Tactic: rw [pow_add (-1 : ℂ), pow_mul (-1 : ℂ), neg_one_sq, one_pow, one_mul, pow_one, ←
        neg_eq_neg_one_mul, ← sub_eq_add_neg]