-- Mathlib/Analysis/SpecialFunctions/Complex/Arg.lean
theorem arg_mul_cos_add_sin_mul_I {r : ℝ} (hr : 0 < r) {θ : ℝ} (hθ : θ ∈ Set.Ioc (-π) π) :
    arg (r * (cos θ + sin θ * I)) = θ

Proof state:
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : θ < -(π / 2)
hθ : -π < θ
hcos : Real.cos θ < 0
⊢ θ < 0

Tactic:
linarith

Result:
no goals

Proof state:
case neg.inr
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : π / 2 < θ
hθ : θ ≤ π
hcos : Real.cos θ < 0
hsin : 0 ≤ Real.sin θ
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Tactic:
rw [if_neg, if_pos, ← Real.sin_sub_pi, Real.arcsin_sin, sub_add_cancel] <;> [linarith;
  linarith; exact hsin; exact hcos.not_le]

Result:
no goals

Proof state:
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
⊢ (↑r * (cos ↑θ + sin ↑θ * I)).arg = θ

Tactic:
simp only [arg, norm_mul, norm_cos_add_sin_mul_I, Complex.norm_of_nonneg hr.le, mul_one]

Result:
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
⊢ (if 0 ≤ (↑r * (cos ↑θ + sin ↑θ * I)).re then Real.arcsin ((↑r * (cos ↑θ + sin ↑θ * I)).im / r)
    else
      if 0 ≤ (↑r * (cos ↑θ + sin ↑θ * I)).im then Real.arcsin ((-(↑r * (cos ↑θ + sin ↑θ * I))).im / r) + π
      else Real.arcsin ((-(↑r * (cos ↑θ + sin ↑θ * I))).im / r) - π) =
    θ

Proof state:
case neg.inr
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : π / 2 < θ
hθ : θ ≤ π
hcos : Real.cos θ < 0
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Tactic:
have hsin : 0 ≤ Real.sin θ := Real.sin_nonneg_of_mem_Icc ⟨by linarith, hθ⟩

Result:
case neg.inr
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : π / 2 < θ
hθ : θ ≤ π
hcos : Real.cos θ < 0
hsin : 0 ≤ Real.sin θ
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Proof state:
case neg.inl
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : θ < -(π / 2)
hθ : -π < θ
hcos : Real.cos θ < 0
hsin : Real.sin θ < 0
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Tactic:
rw [if_neg, if_neg, ← Real.sin_add_pi, Real.arcsin_sin, add_sub_cancel_right] <;> [linarith;
  linarith; exact hsin.not_le; exact hcos.not_le]

Result:
no goals

Proof state:
case neg.inr
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : π / 2 < θ
hθ : θ ≤ π
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Tactic:
have hcos : Real.cos θ < 0 := Real.cos_neg_of_pi_div_two_lt_of_lt h₁ (by linarith)

Result:
case neg.inr
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : π / 2 < θ
hθ : θ ≤ π
hcos : Real.cos θ < 0
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Proof state:
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
⊢ (if 0 ≤ (↑r * (cos ↑θ + sin ↑θ * I)).re then Real.arcsin ((↑r * (cos ↑θ + sin ↑θ * I)).im / r)
    else
      if 0 ≤ (↑r * (cos ↑θ + sin ↑θ * I)).im then Real.arcsin ((-(↑r * (cos ↑θ + sin ↑θ * I))).im / r) + π
      else Real.arcsin ((-(↑r * (cos ↑θ + sin ↑θ * I))).im / r) - π) =
    θ

Tactic:
simp only [re_ofReal_mul, im_ofReal_mul, neg_im, ← ofReal_cos, ← ofReal_sin, ←
  mk_eq_add_mul_I, neg_div, mul_div_cancel_left₀ _ hr.ne', mul_nonneg_iff_right_nonneg_of_pos hr]

Result:
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Proof state:
case neg.inl
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : θ < -(π / 2)
hθ : -π < θ
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Tactic:
have hcos : Real.cos θ < 0 := by
  rw [← neg_pos, ← Real.cos_add_pi]
  refine Real.cos_pos_of_mem_Ioo ⟨?_, ?_⟩ <;> linarith

Result:
case neg.inl
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : θ < -(π / 2)
hθ : -π < θ
hcos : Real.cos θ < 0
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Proof state:
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : θ < -(π / 2)
hθ : -π < θ
⊢ 0 < Real.cos (θ + π)

Tactic:
refine Real.cos_pos_of_mem_Ioo ⟨?_, ?_⟩ <;> linarith

Result:
no goals

Proof state:
case neg
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
h₁ : θ ∉ Icc (-(π / 2)) (π / 2)
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Tactic:
rw [Set.mem_Icc, not_and_or, not_le, not_le] at h₁

Result:
case neg
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
h₁ : θ < -(π / 2) ∨ π / 2 < θ
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Proof state:
case pos
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
h₁ : θ ∈ Icc (-(π / 2)) (π / 2)
⊢ Real.arcsin (Real.sin θ) = θ

case pos.hc
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
h₁ : θ ∈ Icc (-(π / 2)) (π / 2)
⊢ 0 ≤ Real.cos θ

Tactic:
exacts [Real.arcsin_sin' h₁, Real.cos_nonneg_of_mem_Icc h₁]

Result:
no goals

Proof state:
case neg.inl
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
h₁ : θ < -(π / 2)
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Tactic:
replace hθ := hθ.1

Result:
case neg.inl
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : θ < -(π / 2)
hθ : -π < θ
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Proof state:
case neg
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
h₁ : θ < -(π / 2) ∨ π / 2 < θ
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Tactic:
rcases h₁ with h₁ | h₁

Result:
case neg.inl
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
h₁ : θ < -(π / 2)
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

case neg.inr
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
h₁ : π / 2 < θ
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Proof state:
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : π / 2 < θ
hθ : θ ≤ π
hcos : Real.cos θ < 0
⊢ 0 ≤ θ

Tactic:
linarith

Result:
no goals

Proof state:
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : θ < -(π / 2)
hθ : -π < θ
⊢ Real.cos θ < 0

Tactic:
rw [← neg_pos, ← Real.cos_add_pi]

Result:
r : ℝ
hr : 0 < r
θ : ℝ
h₁ : θ < -(π / 2)
hθ : -π < θ
⊢ 0 < Real.cos (θ + π)

Proof state:
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

Tactic:
by_cases h₁ : θ ∈ Set.Icc (-(π / 2)) (π / 2)

Result:
case pos
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
h₁ : θ ∈ Icc (-(π / 2)) (π / 2)
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ

case neg
r : ℝ
hr : 0 < r
θ : ℝ
hθ : θ ∈ Ioc (-π) π
h₁ : θ ∉ Icc (-(π / 2)) (π / 2)
⊢ (if 0 ≤ Real.cos θ then Real.arcsin (Real.sin θ)
    else if 0 ≤ Real.sin θ then Real.arcsin (-Real.sin θ) + π else Real.arcsin (-Real.sin θ) - π) =
    θ