rw [Real.tan_pi_div_two_sub, Real.tan_pi_div_four, one_eq_inv]
x : ℝ
hx : x ∈ Set.Ioo 0 (Real.pi / 2)
y' : ℝ → ℝ := fun x => (x - Real.pi / 4).tan
⊢ (Real.pi / 4).tan = (Real.pi / 2 - Real.pi / 4).tan
no goals