-- Mathlib/CategoryTheory/Abelian/RightDerived.lean
lemma InjectiveResolution.toRightDerivedZero_eq
    {X : C} (I : InjectiveResolution X) (F : C ⥤ D) [F.Additive] :
    F.toRightDerivedZero.app X = I.toRightDerivedZero' F ≫
      (CochainComplex.isoHomologyπ₀ _).hom ≫ (I.isoRightDerivedObj F 0).inv

Proof state:
C : Type u
inst✝⁵ : Category.{v, u} C
D : Type u_1
inst✝⁴ : Category.{u_2, u_1} D
inst✝³ : Abelian C
inst✝² : HasInjectiveResolutions C
inst✝¹ : Abelian D
X : C
I : InjectiveResolution X
F : C ⥤ D
inst✝ : F.Additive
h₁ :
  I.toRightDerivedZero' F =
    (injectiveResolution X).toRightDerivedZero' F ≫
      HomologicalComplex.cyclesMap
        ((F.mapHomologicalComplex (ComplexShape.up ℕ)).map (desc (𝟙 X) I (injectiveResolution X))) 0
h₂ :
  (I.isoRightDerivedToHomotopyCategoryObj F).hom =
    (F.mapHomologicalComplex (ComplexShape.up ℕ) ⋙ HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
      (desc (𝟙 X) I (injectiveResolution X))
⊢ (injectiveResolution X).toRightDerivedZero' F ≫
      ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj (injectiveResolution X).cocomplex).homologyπ 0 ≫
        (HomotopyCategory.homologyFunctorFactors D (ComplexShape.up ℕ) 0).inv.app
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj (injectiveResolution X).cocomplex) ≫
          (HomotopyCategory.homologyFunctor D (ComplexShape.up ℕ) 0).map
            ((F.mapHomologicalComplex (ComplexShape.up ℕ) ⋙ HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
              (desc (𝟙 X) I (injectiveResolution X))) =
    (injectiveResolution X).toRightDerivedZero' F ≫
      ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj (injectiveResolution X).cocomplex).homologyπ 0 ≫
        HomologicalComplex.homologyMap
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).map (desc (𝟙 X) I (injectiveResolution X))) 0 ≫
          (HomotopyCategory.homologyFunctorFactors D (ComplexShape.up ℕ) 0).inv.app
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj I.cocomplex)

Tactic:
erw [← NatTrans.naturality]

Result:
C : Type u
inst✝⁵ : Category.{v, u} C
D : Type u_1
inst✝⁴ : Category.{u_2, u_1} D
inst✝³ : Abelian C
inst✝² : HasInjectiveResolutions C
inst✝¹ : Abelian D
X : C
I : InjectiveResolution X
F : C ⥤ D
inst✝ : F.Additive
h₁ :
  I.toRightDerivedZero' F =
    (injectiveResolution X).toRightDerivedZero' F ≫
      HomologicalComplex.cyclesMap
        ((F.mapHomologicalComplex (ComplexShape.up ℕ)).map (desc (𝟙 X) I (injectiveResolution X))) 0
h₂ :
  (I.isoRightDerivedToHomotopyCategoryObj F).hom =
    (F.mapHomologicalComplex (ComplexShape.up ℕ) ⋙ HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
      (desc (𝟙 X) I (injectiveResolution X))
⊢ (injectiveResolution X).toRightDerivedZero' F ≫
      ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj (injectiveResolution X).cocomplex).homologyπ 0 ≫
        (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) 0).map
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).map (desc (𝟙 X) I (injectiveResolution X))) ≫
          (HomotopyCategory.homologyFunctorFactors D (ComplexShape.up ℕ) 0).inv.app
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj I.cocomplex) =
    (injectiveResolution X).toRightDerivedZero' F ≫
      ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj (injectiveResolution X).cocomplex).homologyπ 0 ≫
        HomologicalComplex.homologyMap
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).map (desc (𝟙 X) I (injectiveResolution X))) 0 ≫
          (HomotopyCategory.homologyFunctorFactors D (ComplexShape.up ℕ) 0).inv.app
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj I.cocomplex)

Proof state:
C : Type u
inst✝⁵ : Category.{v, u} C
D : Type u_1
inst✝⁴ : Category.{u_2, u_1} D
inst✝³ : Abelian C
inst✝² : HasInjectiveResolutions C
inst✝¹ : Abelian D
X : C
I : InjectiveResolution X
F : C ⥤ D
inst✝ : F.Additive
h₁ :
  I.toRightDerivedZero' F =
    (injectiveResolution X).toRightDerivedZero' F ≫
      HomologicalComplex.cyclesMap
        ((F.mapHomologicalComplex (ComplexShape.up ℕ)).map (desc (𝟙 X) I (injectiveResolution X))) 0
⊢ (injectiveResolution X).toRightDerivedZero' F ≫
      ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj (injectiveResolution X).cocomplex).homologyπ 0 ≫
        (HomotopyCategory.homologyFunctorFactors D (ComplexShape.up ℕ) 0).inv.app
          ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj (injectiveResolution X).cocomplex) =
    I.toRightDerivedZero' F ≫
      ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj I.cocomplex).homologyπ 0 ≫
        (HomotopyCategory.homologyFunctorFactors D (ComplexShape.up ℕ) 0).inv.app
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj I.cocomplex) ≫
          (HomotopyCategory.homologyFunctor D (ComplexShape.up ℕ) 0).map (I.isoRightDerivedToHomotopyCategoryObj F).inv

Tactic:
have h₂ : (I.isoRightDerivedToHomotopyCategoryObj F).hom =
  (F.mapHomologicalComplex _ ⋙ HomotopyCategory.quotient _ _).map (desc (𝟙 X) _ _) :=
  comp_id _

Result:
C : Type u
inst✝⁵ : Category.{v, u} C
D : Type u_1
inst✝⁴ : Category.{u_2, u_1} D
inst✝³ : Abelian C
inst✝² : HasInjectiveResolutions C
inst✝¹ : Abelian D
X : C
I : InjectiveResolution X
F : C ⥤ D
inst✝ : F.Additive
h₁ :
  I.toRightDerivedZero' F =
    (injectiveResolution X).toRightDerivedZero' F ≫
      HomologicalComplex.cyclesMap
        ((F.mapHomologicalComplex (ComplexShape.up ℕ)).map (desc (𝟙 X) I (injectiveResolution X))) 0
h₂ :
  (I.isoRightDerivedToHomotopyCategoryObj F).hom =
    (F.mapHomologicalComplex (ComplexShape.up ℕ) ⋙ HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
      (desc (𝟙 X) I (injectiveResolution X))
⊢ (injectiveResolution X).toRightDerivedZero' F ≫
      ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj (injectiveResolution X).cocomplex).homologyπ 0 ≫
        (HomotopyCategory.homologyFunctorFactors D (ComplexShape.up ℕ) 0).inv.app
          ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj (injectiveResolution X).cocomplex) =
    I.toRightDerivedZero' F ≫
      ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj I.cocomplex).homologyπ 0 ≫
        (HomotopyCategory.homologyFunctorFactors D (ComplexShape.up ℕ) 0).inv.app
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj I.cocomplex) ≫
          (HomotopyCategory.homologyFunctor D (ComplexShape.up ℕ) 0).map (I.isoRightDerivedToHomotopyCategoryObj F).inv