-- Mathlib/Analysis/SpecialFunctions/Trigonometric/EulerSineProd.lean
theorem sin_pi_mul_eq (z : ℂ) (n : ℕ) :
    Complex.sin (π * z) =
      ((π * z * ∏ j ∈ Finset.range n, ((1 : ℂ) - z ^ 2 / ((j : ℂ) + 1) ^ 2)) *
          ∫ x in (0 : ℝ)..π / 2, Complex.cos (2 * z * x) * (cos x : ℂ) ^ (2 * n)) /
        (∫ x in (0 : ℝ)..π / 2, cos x ^ (2 * n) : ℝ)

Proof state:
case inr.succ
z : ℂ
hz : z ≠ 0
n : ℕ
A : ℂ := ∏ j ∈ Finset.range n, (1 - z ^ 2 / (↑j + 1) ^ 2)
B : ℂ := ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n)
C : ℝ := ∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * n)
hn : Complex.sin (↑π * z) = ↑π * z * A * B / ↑C
aux' : 2 * n.succ = 2 * n + 2
this✝ : ∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * n.succ) = (2 * ↑n + 1) / (2 * ↑n + 2) * C
this :
  ↑π * z * (A * (1 - z ^ 2 / (↑n + 1) ^ 2)) *
      ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n.succ) =
    ↑π * z * A *
      ((1 - z ^ 2 / ↑n.succ ^ 2) * ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n.succ))
⊢ ↑π * z * A * B / ↑C =
    ↑π * z * A *
        ((1 - z ^ 2 / ↑n.succ ^ 2) * ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n.succ)) /
      ↑((2 * ↑n + 1) / (2 * ↑n + 2) * C)

Tactic:
suffices
  (((1 : ℂ) - z ^ 2 / (n.succ : ℂ) ^ 2) *
      ∫ x in (0 : ℝ)..π / 2, Complex.cos (2 * z * x) * (cos x : ℂ) ^ (2 * n.succ)) =
    (2 * n + 1) / (2 * n + 2) * B by
  rw [this, Complex.ofReal_mul, Complex.ofReal_div]
  have : (C : ℂ) ≠ 0 := Complex.ofReal_ne_zero.mpr (integral_cos_pow_pos _).ne'
  have : 2 * (n : ℂ) + 1 ≠ 0 := by
    convert (Nat.cast_add_one_ne_zero (2 * n) : (↑(2 * n) + 1 : ℂ) ≠ 0)
    simp
  have : 2 * (n : ℂ) + 2 ≠ 0 := by
    convert (Nat.cast_add_one_ne_zero (2 * n + 1) : (↑(2 * n + 1) + 1 : ℂ) ≠ 0) using 1
    push_cast; ring
  field_simp; ring

Result:
case inr.succ
z : ℂ
hz : z ≠ 0
n : ℕ
A : ℂ := ∏ j ∈ Finset.range n, (1 - z ^ 2 / (↑j + 1) ^ 2)
B : ℂ := ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n)
C : ℝ := ∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * n)
hn : Complex.sin (↑π * z) = ↑π * z * A * B / ↑C
aux' : 2 * n.succ = 2 * n + 2
this✝ : ∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * n.succ) = (2 * ↑n + 1) / (2 * ↑n + 2) * C
this :
  ↑π * z * (A * (1 - z ^ 2 / (↑n + 1) ^ 2)) *
      ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n.succ) =
    ↑π * z * A *
      ((1 - z ^ 2 / ↑n.succ ^ 2) * ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n.succ))
⊢ (1 - z ^ 2 / ↑n.succ ^ 2) * ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n.succ) =
    (2 * ↑n + 1) / (2 * ↑n + 2) * B

Proof state:
case inl
n : ℕ
⊢ Complex.sin (↑π * 0) =
    ((↑π * 0 * ∏ j ∈ Finset.range n, (1 - 0 ^ 2 / (↑j + 1) ^ 2)) *
        ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * 0 * ↑x) * ↑(cos x) ^ (2 * n)) /
      ↑(∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * n))

Tactic:
simp

Result:
no goals

Proof state:
case inr.zero
z : ℂ
hz : z ≠ 0
⊢ Complex.sin (↑π * z) =
    ((↑π * z * ∏ j ∈ Finset.range 0, (1 - z ^ 2 / (↑j + 1) ^ 2)) *
        ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * 0)) /
      ↑(∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * 0))

Tactic:
simp_rw [mul_zero, pow_zero, mul_one, Finset.prod_range_zero, mul_one,
  integral_one, sub_zero]

Result:
case inr.zero
z : ℂ
hz : z ≠ 0
⊢ Complex.sin (↑π * z) = (↑π * z * ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x)) / ↑(π / 2)

Proof state:
case inr.succ
z : ℂ
hz : z ≠ 0
n : ℕ
hn :
  Complex.sin (↑π * z) =
    ((↑π * z * ∏ j ∈ Finset.range n, (1 - z ^ 2 / (↑j + 1) ^ 2)) *
        ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n)) /
      ↑(∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * n))
⊢ Complex.sin (↑π * z) =
    ((↑π * z * ∏ j ∈ Finset.range (n + 1), (1 - z ^ 2 / (↑j + 1) ^ 2)) *
        ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * (n + 1))) /
      ↑(∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * (n + 1)))

Tactic:
rw [hn, Finset.prod_range_succ]

Result:
case inr.succ
z : ℂ
hz : z ≠ 0
n : ℕ
hn :
  Complex.sin (↑π * z) =
    ((↑π * z * ∏ j ∈ Finset.range n, (1 - z ^ 2 / (↑j + 1) ^ 2)) *
        ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n)) /
      ↑(∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * n))
⊢ ((↑π * z * ∏ j ∈ Finset.range n, (1 - z ^ 2 / (↑j + 1) ^ 2)) *
        ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * n)) /
      ↑(∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * n)) =
    (↑π * z * ((∏ x ∈ Finset.range n, (1 - z ^ 2 / (↑x + 1) ^ 2)) * (1 - z ^ 2 / (↑n + 1) ^ 2)) *
        ∫ (x : ℝ) in 0 ..π / 2, Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (2 * (n + 1))) /
      ↑(∫ (x : ℝ) in 0 ..π / 2, cos x ^ (2 * (n + 1)))