have h5 : StrictMonoOn y1 (Set.Icc 0 (Real.pi / 2)) := by
  intro a _ b _ hab
  exact sub_lt_sub_right hab _
x : ℝ
hx : x ∈ Set.Ioo 0 (Real.pi / 2)
y' : ℝ → ℝ := fun x => (x - Real.pi / 4).tan
h4 : (Real.pi / 4).tan = y' (Real.pi / 2)
y1 : ℝ → ℝ := fun x => x - Real.pi / 4
⊢ (x - Real.pi / 4).tan < y' (Real.pi / 2)
x : ℝ
hx : x ∈ Set.Ioo 0 (Real.pi / 2)
y' : ℝ → ℝ := fun x => (x - Real.pi / 4).tan
h4 : (Real.pi / 4).tan = y' (Real.pi / 2)
y1 : ℝ → ℝ := fun x => x - Real.pi / 4
h5 : StrictMonoOn y1 (Set.Icc 0 (Real.pi / 2))
⊢ (x - Real.pi / 4).tan < y' (Real.pi / 2)