let y' x := Real.tan (x - Real.pi / 4)
x : ℝ
hx : x ∈ Set.Ioo 0 (Real.pi / 2)
⊢ (x - Real.pi / 4).tan < 1
x : ℝ
hx : x ∈ Set.Ioo 0 (Real.pi / 2)
y' : ℝ → ℝ := fun x => (x - Real.pi / 4).tan
⊢ (x - Real.pi / 4).tan < 1