rw [sub_mul] at this
a b c✝ : ℤα
c x : ℝ
c_pos : c > 0
hc : c * c⁻¹ = 1
h_abs_c : |c| = c
this : |(c⁻¹ * x - ↑(round (c⁻¹ * x))) * c| = |c⁻¹ * x - ↑(round (c⁻¹ * x))| * |c|
⊢ |x - c * ↑(round (c⁻¹ * x))| ≤ 2⁻¹ * c
a b c✝ : ℤα
c x : ℝ
c_pos : c > 0
hc : c * c⁻¹ = 1
h_abs_c : |c| = c
this : |c⁻¹ * x * c - ↑(round (c⁻¹ * x)) * c| = |c⁻¹ * x - ↑(round (c⁻¹ * x))| * |c|
⊢ |x - c * ↑(round (c⁻¹ * x))| ≤ 2⁻¹ * c