rw [div_le_div_iff]
C : Type u_1
J : Type u_2
r : C → J → Prop
a b k : ℕ
ha : 0 < a
hb : 0 < b
⊢ (↑b - 1) / (2 * ↑b) ≤ ↑k / ↑a ↔ (b - 1) * a ≤ k * (2 * b)
C : Type u_1
J : Type u_2
r : C → J → Prop
a b k : ℕ
ha : 0 < a
hb : 0 < b
⊢ (↑b - 1) * ↑a ≤ ↑k * (2 * ↑b) ↔ (b - 1) * a ≤ k * (2 * b)

case b0
C : Type u_1
J : Type u_2
r : C → J → Prop
a b k : ℕ
ha : 0 < a
hb : 0 < b
⊢ 0 < 2 * ↑b

case d0
C : Type u_1
J : Type u_2
r : C → J → Prop
a b k : ℕ
ha : 0 < a
hb : 0 < b
⊢ 0 < ↑a