-- Mathlib/CategoryTheory/Abelian/RightDerived.lean
lemma InjectiveResolution.rightDerivedToHomotopyCategory_app_eq
    {F G : C ⥤ D} [F.Additive] [G.Additive] (α : F ⟶ G) {X : C} (P : InjectiveResolution X) :
    (NatTrans.rightDerivedToHomotopyCategory α).app X =
      (P.isoRightDerivedToHomotopyCategoryObj F).hom ≫
        (HomotopyCategory.quotient _ _).map
          ((NatTrans.mapHomologicalComplex α _).app P.cocomplex) ≫
          (P.isoRightDerivedToHomotopyCategoryObj G).inv

Proof state:
case intro
C : Type u
inst✝⁶ : Category.{v, u} C
D : Type u_1
inst✝⁵ : Category.{u_2, u_1} D
inst✝⁴ : Abelian C
inst✝³ : HasInjectiveResolutions C
inst✝² : Abelian D
F G : C ⥤ D
inst✝¹ : F.Additive
inst✝ : G.Additive
α : F ⟶ G
X : C
P : InjectiveResolution X
β : (injectiveResolution X).cocomplex ⟶ P.cocomplex
hβ : (HomotopyCategory.quotient C (ComplexShape.up ℕ)).map β = P.iso.hom
⊢ (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app ((injectiveResolutions C).obj X).as) ≫
      (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map ((G.mapHomologicalComplex (ComplexShape.up ℕ)).map β) =
    (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map ((F.mapHomologicalComplex (ComplexShape.up ℕ)).map β) ≫
      (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app P.cocomplex)

Tactic:
simp only [← Functor.map_comp, NatTrans.mapHomologicalComplex_naturality]

Result:
case intro
C : Type u
inst✝⁶ : Category.{v, u} C
D : Type u_1
inst✝⁵ : Category.{u_2, u_1} D
inst✝⁴ : Abelian C
inst✝³ : HasInjectiveResolutions C
inst✝² : Abelian D
F G : C ⥤ D
inst✝¹ : F.Additive
inst✝ : G.Additive
α : F ⟶ G
X : C
P : InjectiveResolution X
β : (injectiveResolution X).cocomplex ⟶ P.cocomplex
hβ : (HomotopyCategory.quotient C (ComplexShape.up ℕ)).map β = P.iso.hom
⊢ (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
      ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app ((injectiveResolutions C).obj X).as ≫
        (G.mapHomologicalComplex (ComplexShape.up ℕ)).map β) =
    (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
      ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app (injectiveResolution X).cocomplex ≫
        (G.mapHomologicalComplex (ComplexShape.up ℕ)).map β)

Proof state:
C : Type u
inst✝⁶ : Category.{v, u} C
D : Type u_1
inst✝⁵ : Category.{u_2, u_1} D
inst✝⁴ : Abelian C
inst✝³ : HasInjectiveResolutions C
inst✝² : Abelian D
F G : C ⥤ D
inst✝¹ : F.Additive
inst✝ : G.Additive
α : F ⟶ G
X : C
P : InjectiveResolution X
⊢ (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app ((injectiveResolutions C).obj X).as) ≫
      (G.mapHomotopyCategory (ComplexShape.up ℕ)).map P.iso.hom ≫
        𝟙
          ((HomotopyCategory.quotient D (ComplexShape.up ℕ)).obj
            ((G.mapHomologicalComplex (ComplexShape.up ℕ)).obj P.cocomplex)) =
    ((F.mapHomotopyCategory (ComplexShape.up ℕ)).map P.iso.hom ≫
        𝟙
          ((HomotopyCategory.quotient D (ComplexShape.up ℕ)).obj
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj P.cocomplex))) ≫
      (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app P.cocomplex)

Tactic:
rw [assoc]

Result:
C : Type u
inst✝⁶ : Category.{v, u} C
D : Type u_1
inst✝⁵ : Category.{u_2, u_1} D
inst✝⁴ : Abelian C
inst✝³ : HasInjectiveResolutions C
inst✝² : Abelian D
F G : C ⥤ D
inst✝¹ : F.Additive
inst✝ : G.Additive
α : F ⟶ G
X : C
P : InjectiveResolution X
⊢ (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app ((injectiveResolutions C).obj X).as) ≫
      (G.mapHomotopyCategory (ComplexShape.up ℕ)).map P.iso.hom ≫
        𝟙
          ((HomotopyCategory.quotient D (ComplexShape.up ℕ)).obj
            ((G.mapHomologicalComplex (ComplexShape.up ℕ)).obj P.cocomplex)) =
    (F.mapHomotopyCategory (ComplexShape.up ℕ)).map P.iso.hom ≫
      𝟙
          ((HomotopyCategory.quotient D (ComplexShape.up ℕ)).obj
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj P.cocomplex)) ≫
        (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
          ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app P.cocomplex)

Proof state:
C : Type u
inst✝⁶ : Category.{v, u} C
D : Type u_1
inst✝⁵ : Category.{u_2, u_1} D
inst✝⁴ : Abelian C
inst✝³ : HasInjectiveResolutions C
inst✝² : Abelian D
F G : C ⥤ D
inst✝¹ : F.Additive
inst✝ : G.Additive
α : F ⟶ G
X : C
P : InjectiveResolution X
⊢ (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app ((injectiveResolutions C).obj X).as) ≫
      (G.mapHomotopyCategory (ComplexShape.up ℕ)).map P.iso.hom ≫
        𝟙
          ((HomotopyCategory.quotient D (ComplexShape.up ℕ)).obj
            ((G.mapHomologicalComplex (ComplexShape.up ℕ)).obj P.cocomplex)) =
    (F.mapHomotopyCategory (ComplexShape.up ℕ)).map P.iso.hom ≫
      𝟙
          ((HomotopyCategory.quotient D (ComplexShape.up ℕ)).obj
            ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj P.cocomplex)) ≫
        (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
          ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app P.cocomplex)

Tactic:
erw [id_comp, comp_id]

Result:
C : Type u
inst✝⁶ : Category.{v, u} C
D : Type u_1
inst✝⁵ : Category.{u_2, u_1} D
inst✝⁴ : Abelian C
inst✝³ : HasInjectiveResolutions C
inst✝² : Abelian D
F G : C ⥤ D
inst✝¹ : F.Additive
inst✝ : G.Additive
α : F ⟶ G
X : C
P : InjectiveResolution X
⊢ (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app ((injectiveResolutions C).obj X).as) ≫
      (G.mapHomotopyCategory (ComplexShape.up ℕ)).map P.iso.hom =
    (F.mapHomotopyCategory (ComplexShape.up ℕ)).map P.iso.hom ≫
      (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app P.cocomplex)

Proof state:
case intro
C : Type u
inst✝⁶ : Category.{v, u} C
D : Type u_1
inst✝⁵ : Category.{u_2, u_1} D
inst✝⁴ : Abelian C
inst✝³ : HasInjectiveResolutions C
inst✝² : Abelian D
F G : C ⥤ D
inst✝¹ : F.Additive
inst✝ : G.Additive
α : F ⟶ G
X : C
P : InjectiveResolution X
β : (injectiveResolution X).cocomplex ⟶ P.cocomplex
hβ : (HomotopyCategory.quotient C (ComplexShape.up ℕ)).map β = P.iso.hom
⊢ (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app ((injectiveResolutions C).obj X).as) ≫
      (G.mapHomotopyCategory (ComplexShape.up ℕ)).map P.iso.hom =
    (F.mapHomotopyCategory (ComplexShape.up ℕ)).map P.iso.hom ≫
      (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app P.cocomplex)

Tactic:
rw [← hβ]

Result:
case intro
C : Type u
inst✝⁶ : Category.{v, u} C
D : Type u_1
inst✝⁵ : Category.{u_2, u_1} D
inst✝⁴ : Abelian C
inst✝³ : HasInjectiveResolutions C
inst✝² : Abelian D
F G : C ⥤ D
inst✝¹ : F.Additive
inst✝ : G.Additive
α : F ⟶ G
X : C
P : InjectiveResolution X
β : (injectiveResolution X).cocomplex ⟶ P.cocomplex
hβ : (HomotopyCategory.quotient C (ComplexShape.up ℕ)).map β = P.iso.hom
⊢ (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app ((injectiveResolutions C).obj X).as) ≫
      (G.mapHomotopyCategory (ComplexShape.up ℕ)).map ((HomotopyCategory.quotient C (ComplexShape.up ℕ)).map β) =
    (F.mapHomotopyCategory (ComplexShape.up ℕ)).map ((HomotopyCategory.quotient C (ComplexShape.up ℕ)).map β) ≫
      (HomotopyCategory.quotient D (ComplexShape.up ℕ)).map
        ((NatTrans.mapHomologicalComplex α (ComplexShape.up ℕ)).app P.cocomplex)