θ : ℂ
⊢ (∃ k, θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2) ↔ ∃ k, θ = ↑k * ↑π
θ : ℂ
⊢ sin θ = 0 ↔ ∃ k, θ = ↑k * ↑π
rw [← Complex.cos_sub_pi_div_two, cos_eq_zero_iff]
case mp
θ : ℂ
⊢ (∃ k, θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2) → ∃ k, θ = ↑k * ↑π

case mpr
θ : ℂ
⊢ (∃ k, θ = ↑k * ↑π) → ∃ k, θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2
θ : ℂ
⊢ (∃ k, θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2) ↔ ∃ k, θ = ↑k * ↑π
constructor
case mp.intro
θ : ℂ
k : ℤ
hk : θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2
⊢ ∃ k, θ = ↑k * ↑π
case mp
θ : ℂ
⊢ (∃ k, θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2) → ∃ k, θ = ↑k * ↑π
rintro ⟨k, hk⟩
case mp.intro
θ : ℂ
k : ℤ
hk : θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2
⊢ θ = ↑(k + 1) * ↑π
case mp.intro
θ : ℂ
k : ℤ
hk : θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2
⊢ ∃ k, θ = ↑k * ↑π
use k + 1
case mp.intro
θ : ℂ
k : ℤ
hk : θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2
⊢ (2 * ↑k + 1) * ↑π + ↑π = (↑k + 1) * ↑π * 2
case mp.intro
θ : ℂ
k : ℤ
hk : θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2
⊢ θ = ↑(k + 1) * ↑π
field_simp [eq_add_of_sub_eq hk]
no goals
case mp.intro
θ : ℂ
k : ℤ
hk : θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2
⊢ (2 * ↑k + 1) * ↑π + ↑π = (↑k + 1) * ↑π * 2
ring
case mpr.intro
k : ℤ
⊢ ∃ k_1, ↑k * ↑π - ↑π / 2 = (2 * ↑k_1 + 1) * ↑π / 2
case mpr
θ : ℂ
⊢ (∃ k, θ = ↑k * ↑π) → ∃ k, θ - ↑π / 2 = (2 * ↑k + 1) * ↑π / 2
rintro ⟨k, rfl⟩
case mpr.intro
k : ℤ
⊢ ↑k * ↑π - ↑π / 2 = (2 * ↑(k - 1) + 1) * ↑π / 2
case mpr.intro
k : ℤ
⊢ ∃ k_1, ↑k * ↑π - ↑π / 2 = (2 * ↑k_1 + 1) * ↑π / 2
use k - 1
case mpr.intro
k : ℤ
⊢ ↑k * ↑π * 2 - ↑π = (2 * (↑k - 1) + 1) * ↑π
case mpr.intro
k : ℤ
⊢ ↑k * ↑π - ↑π / 2 = (2 * ↑(k - 1) + 1) * ↑π / 2
field_simp
no goals
case mpr.intro
k : ℤ
⊢ ↑k * ↑π * 2 - ↑π = (2 * (↑k - 1) + 1) * ↑π
ring