theorem extracted_formal_statement_55.{u_1} {E : Type u_1} [inst : NormedAddCommGroup E] [inst_1 : InnerProductSpace ℝ E]
  [inst_2 : CompleteSpace E] {f : E → ℝ} {f' : E → E} {xm x₀ : E} {γ : ℕ+ → ℝ} {alg : Nesterov f f' γ x₀}
  (hfun : ConvexOn ℝ univ f) (hg : ∀ (k : ℕ+), γ k = 2 / (↑↑k + 1)) (min : IsMinOn f univ xm) (k : ℕ+)
  (h4 :
    1 / γ k ^ 2 * (f (Nesterov.x f f' γ x₀ ↑k) - f xm) +
        ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.v f f' γ x₀ ↑k - xm‖ ^ 2 ≤
      (1 - γ k) / γ k ^ 2 * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm) +
        ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖ ^ 2)
  (con : (1 - γ k) / γ k ^ 2 ≤ 1 / γ (k - 1) ^ 2) (this_1 : γ k ^ 2 > 0)
  (this : f xm ≤ f (Nesterov.x f f' γ x₀ (↑k - 1)))
  (h_original_goal :
    (1 - γ k) / γ k ^ 2 * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm) ≤
      1 / γ (k - 1) ^ 2 * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm)) :
  (1 - γ k) / γ k ^ 2 ≤ 1 / γ (k - 1) ^ 2 := sorry

State before:
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : InnerProductSpace ℝ E
inst✝ : CompleteSpace E
f : E → ℝ
f' : E → E
xm x₀ : E
γ : ℕ+ → ℝ
alg : Nesterov f f' γ x₀
hfun : ConvexOn ℝ univ f
hg : ∀ (k : ℕ+), γ k = 2 / (↑↑k + 1)
min : IsMinOn f univ xm
k : ℕ+
h4 :
  1 / γ k ^ 2 * (f (Nesterov.x f f' γ x₀ ↑k) - f xm) +
      ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.v f f' γ x₀ ↑k - xm‖ ^ 2 ≤
    (1 - γ k) / γ k ^ 2 * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm) +
      ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖ ^ 2
con : (1 - γ k) / γ k ^ 2 ≤ 1 / γ (k - 1) ^ 2
this✝ : γ k ^ 2 > 0
this : f xm ≤ f (Nesterov.x f f' γ x₀ (↑k - 1))
⊢ (1 - γ k) / γ k ^ 2 * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm) ≤
    1 / γ (k - 1) ^ 2 * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm)

Tactic: apply mul_le_mul_of_nonneg_right _ (by linarith)

State after:
[E : Type u_1
 inst✝² : NormedAddCommGroup E
 inst✝¹ : InnerProductSpace ℝ E
 inst✝ : CompleteSpace E
 f : E → ℝ
 f' : E → E
 xm x₀ : E
 γ : ℕ+ → ℝ
 alg : Nesterov f f' γ x₀
 hfun : ConvexOn ℝ univ f
 hg : ∀ (k : ℕ+), γ k = 2 / (↑↑k + 1)
 min : IsMinOn f univ xm
 k : ℕ+
 h4 :
   1 / γ k ^ 2 * (f (Nesterov.x f f' γ x₀ ↑k) - f xm) +
       ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.v f f' γ x₀ ↑k - xm‖ ^ 2 ≤
     (1 - γ k) / γ k ^ 2 * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm) +
       ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖ ^ 2
 con : (1 - γ k) / γ k ^ 2 ≤ 1 / γ (k - 1) ^ 2
 this✝ : γ k ^ 2 > 0
 this : f xm ≤ f (Nesterov.x f f' γ x₀ (↑k - 1))
 ⊢ (1 - γ k) / γ k ^ 2 ≤ 1 / γ (k - 1) ^ 2]

Relevant lemmas:
mul_le_mul_of_nonneg_right : ∀ {α : Type u_3} {a b c : α} [inst : Mul α] [inst_3 : Zero α] [inst_4 : Preorder α]
  [inst_5 : MulPosMono α], b ≤ c → 0 ≤ a → b * a ≤ c * a