rw [mul_le_mul_right c_pos]
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))| ≤ 1 / 2
⊢ |c⁻¹ * x - ↑(round (c⁻¹ * x))| * c ≤ 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 - ↑(round (c⁻¹ * x))| ≤ 1 / 2
⊢ |c⁻¹ * x - ↑(round (c⁻¹ * x))| ≤ 2⁻¹