theorem extracted_formal_statement_48.{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} {alg : Gradient_Descent_fix_stepsize f f' x₀}
  (hfun : ConvexOn ℝ univ f)
  (step₂ : Gradient_Descent_fix_stepsize.a f f' x₀ ≤ 1 / ↑(Gradient_Descent_fix_stepsize.l f f' x₀)) (k : ℕ)
  (step1₂ : ↑(Gradient_Descent_fix_stepsize.l f f' x₀) ≤ 1 / Gradient_Descent_fix_stepsize.a f f' x₀)
  (this_1 : LipschitzWith (Gradient_Descent_fix_stepsize.l f f' x₀) f')
  (this : Gradient_Descent_fix_stepsize.l f f' x₀ > 0)
  (tα : 1 / (2 * (↑k + 1) * Gradient_Descent_fix_stepsize.a f f' x₀) > 0)
  (mono :
    ∀ (k : ℕ), f (Gradient_Descent_fix_stepsize.x f f' x₀ (k + 1)) ≤ f (Gradient_Descent_fix_stepsize.x f f' x₀ k))
  (j : ℕ)
  (IH :
    ∑ k ∈ Finset.range (j + 1), (f (Gradient_Descent_fix_stepsize.x f f' x₀ (k + 1)) - f xm) ≤
      1 / (2 * Gradient_Descent_fix_stepsize.a f f' x₀) *
        (‖x₀ - xm‖ ^ 2 - ‖Gradient_Descent_fix_stepsize.x f f' x₀ (j + 1) - xm‖ ^ 2))
  (xdescent :
    f (Gradient_Descent_fix_stepsize.x f f' x₀ (j + 1 + 1)) ≤
      f xm +
        1 / (2 * Gradient_Descent_fix_stepsize.a f f' x₀) *
          (‖Gradient_Descent_fix_stepsize.x f f' x₀ (j + 1) - xm‖ ^ 2 -
            ‖Gradient_Descent_fix_stepsize.x f f' x₀ (j + 1 + 1) - xm‖ ^ 2)) :
  ∑ x ∈ Finset.range (1 + j), (f (Gradient_Descent_fix_stepsize.x f f' x₀ (x + 1)) - f xm) +
      (f (Gradient_Descent_fix_stepsize.x f f' x₀ (2 + j)) - f xm) =
    ∑ x ∈ Finset.range (1 + j), (f (Gradient_Descent_fix_stepsize.x f f' x₀ (x + 1)) - f xm) +
        f (Gradient_Descent_fix_stepsize.x f f' x₀ (2 + j)) -
      f xm := 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
alg : Gradient_Descent_fix_stepsize f f' x₀
hfun : ConvexOn ℝ univ f
step₂ : Gradient_Descent_fix_stepsize.a f f' x₀ ≤ 1 / ↑(Gradient_Descent_fix_stepsize.l f f' x₀)
k : ℕ
step1₂ : ↑(Gradient_Descent_fix_stepsize.l f f' x₀) ≤ 1 / Gradient_Descent_fix_stepsize.a f f' x₀
this✝ : LipschitzWith (Gradient_Descent_fix_stepsize.l f f' x₀) f'
this : Gradient_Descent_fix_stepsize.l f f' x₀ > 0
tα : 1 / (2 * (↑k + 1) * Gradient_Descent_fix_stepsize.a f f' x₀) > 0
mono : ∀ (k : ℕ), f (Gradient_Descent_fix_stepsize.x f f' x₀ (k + 1)) ≤ f (Gradient_Descent_fix_stepsize.x f f' x₀ k)
j : ℕ
IH :
  ∑ k ∈ Finset.range (j + 1), (f (Gradient_Descent_fix_stepsize.x f f' x₀ (k + 1)) - f xm) ≤
    1 / (2 * Gradient_Descent_fix_stepsize.a f f' x₀) *
      (‖x₀ - xm‖ ^ 2 - ‖Gradient_Descent_fix_stepsize.x f f' x₀ (j + 1) - xm‖ ^ 2)
xdescent :
  f (Gradient_Descent_fix_stepsize.x f f' x₀ (j + 1 + 1)) ≤
    f xm +
      1 / (2 * Gradient_Descent_fix_stepsize.a f f' x₀) *
        (‖Gradient_Descent_fix_stepsize.x f f' x₀ (j + 1) - xm‖ ^ 2 -
          ‖Gradient_Descent_fix_stepsize.x f f' x₀ (j + 1 + 1) - xm‖ ^ 2)
⊢ ∑ x ∈ Finset.range (1 + j), (f (Gradient_Descent_fix_stepsize.x f f' x₀ (x + 1)) - f xm) +
      (f (Gradient_Descent_fix_stepsize.x f f' x₀ (2 + j)) - f xm) =
    ∑ x ∈ Finset.range (1 + j), (f (Gradient_Descent_fix_stepsize.x f f' x₀ (x + 1)) - f xm) +
        f (Gradient_Descent_fix_stepsize.x f f' x₀ (2 + j)) -
      f xm

Tactic: rw [add_sub]

State after:

Relevant lemmas:
add_sub : ∀ {G : Type u_3} [inst : SubNegMonoid G] (a b c : G), a + (b - c) = a + b - c