theorem extracted_formal_statement_25.{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)
  (k : ℕ+)
  (this_1 :
    f (Nesterov.x f f' γ x₀ ↑k) - f xm - (1 - γ k) * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm) =
      γ k * (f (Nesterov.x f f' γ x₀ ↑k) - f xm) +
        (1 - γ k) * (f (Nesterov.x f f' γ x₀ ↑k) - f (Nesterov.x f f' γ x₀ (↑k - 1))))
  (lzero : 0 < ↑↑k + 1) (hz : γ k ≥ 0) (hl : γ k ≤ 1)
  (this :
    γ k • (xm - Nesterov.x f f' γ x₀ ↑k) + (1 - γ k) • (Nesterov.x f f' γ x₀ (↑k - 1) - Nesterov.x 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) *
          inner (Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k) (γ k • (xm - Nesterov.x f f' γ x₀ ↑k)) +
        ↑(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) - 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 =
    ↑(Nesterov.l f f' γ x₀) *
        inner (Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k)
          (γ k • (xm - Nesterov.x f f' γ x₀ ↑k) +
            (1 - γ k) • (Nesterov.x f f' γ x₀ (↑k - 1) - 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 := 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
k : ℕ+
this✝ :
  f (Nesterov.x f f' γ x₀ ↑k) - f xm - (1 - γ k) * (f (Nesterov.x f f' γ x₀ (↑k - 1)) - f xm) =
    γ k * (f (Nesterov.x f f' γ x₀ ↑k) - f xm) +
      (1 - γ k) * (f (Nesterov.x f f' γ x₀ ↑k) - f (Nesterov.x f f' γ x₀ (↑k - 1)))
lzero : 0 < ↑↑k + 1
hz : γ k ≥ 0
hl : γ k ≤ 1
this :
  γ k • (xm - Nesterov.x f f' γ x₀ ↑k) + (1 - γ k) • (Nesterov.x f f' γ x₀ (↑k - 1) - Nesterov.x 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) *
          inner (Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k) (γ k • (xm - Nesterov.x f f' γ x₀ ↑k)) +
        ↑(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) - 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 =
    ↑(Nesterov.l f f' γ x₀) *
        inner (Nesterov.x f f' γ x₀ ↑k - Nesterov.y f f' γ x₀ k)
          (γ k • (xm - Nesterov.x f f' γ x₀ ↑k) +
            (1 - γ k) • (Nesterov.x f f' γ x₀ (↑k - 1) - 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

Tactic: rw [← mul_add, ← inner_add_right (alg.x k - alg.y k)]

State after:

Relevant lemmas:
mul_add : ∀ {R : Type v} [inst : Mul R] [inst_3 : Add R] [inst_4 : LeftDistribClass R] (a b c : R),
  a * (b + c) = a * b + a * c
inner_add_right : ∀ {𝕜 : Type u_1} {E : Type u_2} [inst : RCLike 𝕜] [inst_3 : SeminormedAddCommGroup E]
  [inst_4 : InnerProductSpace 𝕜 E] (x y z : E), inner x (y + z) = inner x y + inner x z