-- Mathlib/LinearAlgebra/TensorProduct/RightExactness.lean
/-- The ideal of `A ⊗[R] B` generated by `I` is the image of `I ⊗[R] B` -/
lemma Ideal.map_includeLeft_eq (I : Ideal A) :
    (I.map (Algebra.TensorProduct.includeLeft : A →ₐ[R] A ⊗[R] B)).restrictScalars R
      = LinearMap.range (LinearMap.rTensor B (Submodule.subtype (I.restrictScalars R)))

Proof state:
case a.refine_4.intro.refl.add.intro
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_2
B : Type u_3
inst✝³ : Semiring A
inst✝² : Semiring B
inst✝¹ : Algebra R A
inst✝ : Algebra R B
I : Ideal A
x✝ : A ⊗[R] B
hx : x✝ ∈ Submodule.span (A ⊗[R] B) (⇑includeLeft '' ↑I)
x : ↥(Submodule.restrictScalars R I) ⊗[R] B
a b : A ⊗[R] B
hb :
  ∃ y,
    (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y =
      b • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x
x' : ↥(Submodule.restrictScalars R I) ⊗[R] B
ha' :
  (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x' =
    a • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x
⊢ ∃ y,
    (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y =
      (a + b) • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x

Tactic:
obtain ⟨y', hb'⟩ := hb

Result:
case a.refine_4.intro.refl.add.intro.intro
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_2
B : Type u_3
inst✝³ : Semiring A
inst✝² : Semiring B
inst✝¹ : Algebra R A
inst✝ : Algebra R B
I : Ideal A
x✝ : A ⊗[R] B
hx : x✝ ∈ Submodule.span (A ⊗[R] B) (⇑includeLeft '' ↑I)
x : ↥(Submodule.restrictScalars R I) ⊗[R] B
a b : A ⊗[R] B
x' : ↥(Submodule.restrictScalars R I) ⊗[R] B
ha' :
  (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x' =
    a • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x
y' : ↥(Submodule.restrictScalars R I) ⊗[R] B
hb' :
  (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y' =
    b • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x
⊢ ∃ y,
    (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y =
      (a + b) • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x

Proof state:
case h
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_2
B : Type u_3
inst✝³ : Semiring A
inst✝² : Semiring B
inst✝¹ : Algebra R A
inst✝ : Algebra R B
I : Ideal A
x : A ⊗[R] B
hx : x ∈ Submodule.span (A ⊗[R] B) (⇑includeLeft '' ↑I)
⊢ (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) 0 = 0

Tactic:
simp only [map_zero]

Result:
no goals

Proof state:
case a.refine_4.intro.refl.tmul.add
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_2
B : Type u_3
inst✝³ : Semiring A
inst✝² : Semiring B
inst✝¹ : Algebra R A
inst✝ : Algebra R B
I : Ideal A
x✝ : A ⊗[R] B
hx✝ : x✝ ∈ Submodule.span (A ⊗[R] B) (⇑includeLeft '' ↑I)
a : A
b : B
x y : ↥(Submodule.restrictScalars R I) ⊗[R] B
hx :
  ∃ y,
    (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y =
      a ⊗ₜ[R] b • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x
hy :
  ∃ y_1,
    (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y_1 =
      a ⊗ₜ[R] b • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y
⊢ ∃ y_1,
    (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y_1 =
      a ⊗ₜ[R] b • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) (x + y)

Tactic:
obtain ⟨x', hx'⟩ := hx

Result:
case a.refine_4.intro.refl.tmul.add.intro
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_2
B : Type u_3
inst✝³ : Semiring A
inst✝² : Semiring B
inst✝¹ : Algebra R A
inst✝ : Algebra R B
I : Ideal A
x✝ : A ⊗[R] B
hx : x✝ ∈ Submodule.span (A ⊗[R] B) (⇑includeLeft '' ↑I)
a : A
b : B
x y : ↥(Submodule.restrictScalars R I) ⊗[R] B
hy :
  ∃ y_1,
    (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y_1 =
      a ⊗ₜ[R] b • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y
x' : ↥(Submodule.restrictScalars R I) ⊗[R] B
hx' :
  (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x' =
    a ⊗ₜ[R] b • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) x
⊢ ∃ y_1,
    (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) y_1 =
      a ⊗ₜ[R] b • (LinearMap.rTensor B (Submodule.restrictScalars R I).subtype) (x + y)