theorem extracted_formal_statement_32.{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))
  (h2 :
    ∀ (k : ℕ+) (x' : E),
      f (Nesterov.x f f' γ x₀ ↑k) - f x' ≤
        ↑(Nesterov.l f f' γ x₀) *
            inner (Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k) (x' - Nesterov.x f f' γ x₀ ↑k) +
          ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k‖ ^ 2)
  (h3 :
    ∀ (k : ℕ+),
      f (Nesterov.x f f' γ x₀ ↑k) - f xm - (1 - γ k) * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm) ≤
        ↑(Nesterov.l f f' γ x₀) *
            inner (Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k)
              ((1 - γ k) • Nesterov.x f f' γ x₀ (↑k - 1) + γ k • xm - Nesterov.x f f' γ x₀ ↑k) +
          ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k‖ ^ 2)
  (k : ℕ+) (hz : γ k ≥ 0) (hzs : γ k > 0)
  (h_original_goal :
    γ k ^ 2 * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖ ^ 2 - γ k ^ 2 * ‖Nesterov.v f f' γ x₀ ↑k - xm‖ ^ 2 =
        (|γ k| * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖) ^ 2 - (|γ k| * ‖Nesterov.v f f' γ x₀ ↑k - xm‖) ^ 2 ∨
      Nesterov.l f f' γ x₀ = 0) :
  γ k ^ 2 * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖ ^ 2 - γ k ^ 2 * ‖Nesterov.v f f' γ x₀ ↑k - xm‖ ^ 2 =
      (γ k * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖) ^ 2 - (γ k * ‖Nesterov.v f f' γ x₀ ↑k - xm‖) ^ 2 ∨
    Nesterov.l f f' γ x₀ = 0 := 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)
h2 :
  ∀ (k : ℕ+) (x' : E),
    f (Nesterov.x f f' γ x₀ ↑k) - f x' ≤
      ↑(Nesterov.l f f' γ x₀) *
          inner (Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k) (x' - Nesterov.x f f' γ x₀ ↑k) +
        ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k‖ ^ 2
h3 :
  ∀ (k : ℕ+),
    f (Nesterov.x f f' γ x₀ ↑k) - f xm - (1 - γ k) * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm) ≤
      ↑(Nesterov.l f f' γ x₀) *
          inner (Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k)
            ((1 - γ k) • Nesterov.x f f' γ x₀ (↑k - 1) + γ k • xm - Nesterov.x f f' γ x₀ ↑k) +
        ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k‖ ^ 2
k : ℕ+
hz : γ k ≥ 0
hzs : γ k > 0
⊢ γ k ^ 2 * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖ ^ 2 - γ k ^ 2 * ‖Nesterov.v f f' γ x₀ ↑k - xm‖ ^ 2 =
      (|γ k| * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖) ^ 2 - (|γ k| * ‖Nesterov.v f f' γ x₀ ↑k - xm‖) ^ 2 ∨
    Nesterov.l f f' γ x₀ = 0

Tactic: rw [abs_of_nonneg hz]

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)
 h2 :
   ∀ (k : ℕ+) (x' : E),
     f (Nesterov.x f f' γ x₀ ↑k) - f x' ≤
       ↑(Nesterov.l f f' γ x₀) *
           inner (Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k) (x' - Nesterov.x f f' γ x₀ ↑k) +
         ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k‖ ^ 2
 h3 :
   ∀ (k : ℕ+),
     f (Nesterov.x f f' γ x₀ ↑k) - f xm - (1 - γ k) * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm) ≤
       ↑(Nesterov.l f f' γ x₀) *
           inner (Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k)
             ((1 - γ k) • Nesterov.x f f' γ x₀ (↑k - 1) + γ k • xm - Nesterov.x f f' γ x₀ ↑k) +
         ↑(Nesterov.l f f' γ x₀) / 2 * ‖Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k‖ ^ 2
 k : ℕ+
 hz : γ k ≥ 0
 hzs : γ k > 0
 ⊢ γ k ^ 2 * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖ ^ 2 - γ k ^ 2 * ‖Nesterov.v f f' γ x₀ ↑k - xm‖ ^ 2 =
       (γ k * ‖Nesterov.v f f' γ x₀ (↑k - 1) - xm‖) ^ 2 - (γ k * ‖Nesterov.v f f' γ x₀ ↑k - xm‖) ^ 2 ∨
     Nesterov.l f f' γ x₀ = 0]

Relevant lemmas:
abs_of_nonneg : ∀ {α : Type u_1} [inst : Lattice α] [inst_3 : AddGroup α] {a : α} [inst_4 : AddLeftMono α],
  0 ≤ a → |a| = a