-- Mathlib/Analysis/Calculus/MeanValue.lean
theorem image_le_of_liminf_slope_right_lt_deriv_boundary' {f f' : ℝ → ℝ} {a b : ℝ}
    (hf : ContinuousOn f (Icc a b))
    -- `hf'` actually says `liminf (f z - f x) / (z - x) ≤ f' x`
    (hf' : ∀ x ∈ Ico a b, ∀ r, f' x < r → ∃ᶠ z in 𝓝[>] x, slope f x z < r)
    {B B' : ℝ → ℝ} (ha : f a ≤ B a) (hB : ContinuousOn B (Icc a b))
    (hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x)
    (bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x) : ∀ ⦃x⦄, x ∈ Icc a b → f x ≤ B x

Proof state:
case intro.inr.intro.intro.intro.intro.intro
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
s : Set ℝ := {x | f x ≤ B x} ∩ Icc a b
A : ContinuousOn (fun x => (f x, B x)) (Icc a b)
this : IsClosed s
x : ℝ
hxB✝ : f x ≤ B x
xab : x ∈ Ico a b
y : ℝ
hy : y ∈ Ioi x
hxB : f x = B x
r : ℝ
hfr : f' x < r
hrB : r < B' x
hf' : ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
HB : ∀ᶠ (z : ℝ) in 𝓝[>] x, r < slope B x z
z : ℝ
hfz : slope f x z < r
hzB : r < slope B x z
hz : z ∈ Ioc x y
⊢ ({x | f x ≤ B x} ∩ Ioc x y).Nonempty

Tactic:
refine ⟨z, ?_, hz⟩

Result:
case intro.inr.intro.intro.intro.intro.intro
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
s : Set ℝ := {x | f x ≤ B x} ∩ Icc a b
A : ContinuousOn (fun x => (f x, B x)) (Icc a b)
this : IsClosed s
x : ℝ
hxB✝ : f x ≤ B x
xab : x ∈ Ico a b
y : ℝ
hy : y ∈ Ioi x
hxB : f x = B x
r : ℝ
hfr : f' x < r
hrB : r < B' x
hf' : ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
HB : ∀ᶠ (z : ℝ) in 𝓝[>] x, r < slope B x z
z : ℝ
hfz : slope f x z < r
hzB : r < slope B x z
hz : z ∈ Ioc x y
⊢ z ∈ {x | f x ≤ B x}

Proof state:
case intro.inl
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
hf' : ∀ x ∈ Ico a b, ∀ (r : ℝ), f' x < r → ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
s : Set ℝ := {x | f x ≤ B x} ∩ Icc a b
A : ContinuousOn (fun x => (f x, B x)) (Icc a b)
this : IsClosed s
x : ℝ
hxB✝ : f x ≤ B x
xab : x ∈ Ico a b
y : ℝ
hy : y ∈ Ioi x
hxB : f x < B x
⊢ {x | f x ≤ B x} ∈ 𝓝[>] x

Tactic:
have : ∀ᶠ x in 𝓝[Icc a b] x, f x < B x :=
  A x (Ico_subset_Icc_self xab) (IsOpen.mem_nhds (isOpen_lt continuous_fst continuous_snd) hxB)

Result:
case intro.inl
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
hf' : ∀ x ∈ Ico a b, ∀ (r : ℝ), f' x < r → ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
s : Set ℝ := {x | f x ≤ B x} ∩ Icc a b
A : ContinuousOn (fun x => (f x, B x)) (Icc a b)
this✝ : IsClosed s
x : ℝ
hxB✝ : f x ≤ B x
xab : x ∈ Ico a b
y : ℝ
hy : y ∈ Ioi x
hxB : f x < B x
this : ∀ᶠ (x : ℝ) in 𝓝[Icc a b] x, f x < B x
⊢ {x | f x ≤ B x} ∈ 𝓝[>] x

Proof state:
case intro.inr.intro.intro.intro.intro.intro
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
s : Set ℝ := {x | f x ≤ B x} ∩ Icc a b
A : ContinuousOn (fun x => (f x, B x)) (Icc a b)
this : IsClosed s
x : ℝ
hxB✝ : f x ≤ B x
xab : x ∈ Ico a b
y : ℝ
hy : y ∈ Ioi x
hxB : f x = B x
r : ℝ
hfr : f' x < r
hrB : r < B' x
hf' : ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
HB : ∀ᶠ (z : ℝ) in 𝓝[>] x, r < slope B x z
z : ℝ
hfz : slope f x z < r
hzB : r < slope B x z
hz : z ∈ Ioc x y
⊢ z ∈ {x | f x ≤ B x}

Tactic:
have := (hfz.trans hzB).le

Result:
case intro.inr.intro.intro.intro.intro.intro
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
s : Set ℝ := {x | f x ≤ B x} ∩ Icc a b
A : ContinuousOn (fun x => (f x, B x)) (Icc a b)
this✝ : IsClosed s
x : ℝ
hxB✝ : f x ≤ B x
xab : x ∈ Ico a b
y : ℝ
hy : y ∈ Ioi x
hxB : f x = B x
r : ℝ
hfr : f' x < r
hrB : r < B' x
hf' : ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
HB : ∀ᶠ (z : ℝ) in 𝓝[>] x, r < slope B x z
z : ℝ
hfz : slope f x z < r
hzB : r < slope B x z
hz : z ∈ Ioc x y
this : slope f x z ≤ slope B x z
⊢ z ∈ {x | f x ≤ B x}

Proof state:
case intro.inr
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
hf' : ∀ x ∈ Ico a b, ∀ (r : ℝ), f' x < r → ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
s : Set ℝ := {x | f x ≤ B x} ∩ Icc a b
A : ContinuousOn (fun x => (f x, B x)) (Icc a b)
this : IsClosed s
x : ℝ
hxB✝ : f x ≤ B x
xab : x ∈ Ico a b
y : ℝ
hy : y ∈ Ioi x
hxB : f x = B x
⊢ ({x | f x ≤ B x} ∩ Ioc x y).Nonempty

Tactic:
rcases exists_between (bound x xab hxB) with ⟨r, hfr, hrB⟩

Result:
case intro.inr.intro.intro
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
hf' : ∀ x ∈ Ico a b, ∀ (r : ℝ), f' x < r → ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
s : Set ℝ := {x | f x ≤ B x} ∩ Icc a b
A : ContinuousOn (fun x => (f x, B x)) (Icc a b)
this : IsClosed s
x : ℝ
hxB✝ : f x ≤ B x
xab : x ∈ Ico a b
y : ℝ
hy : y ∈ Ioi x
hxB : f x = B x
r : ℝ
hfr : f' x < r
hrB : r < B' x
⊢ ({x | f x ≤ B x} ∩ Ioc x y).Nonempty

Proof state:
case intro.inr.intro.intro
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
s : Set ℝ := {x | f x ≤ B x} ∩ Icc a b
A : ContinuousOn (fun x => (f x, B x)) (Icc a b)
this : IsClosed s
x : ℝ
hxB✝ : f x ≤ B x
xab : x ∈ Ico a b
y : ℝ
hy : y ∈ Ioi x
hxB : f x = B x
r : ℝ
hfr : f' x < r
hrB : r < B' x
hf' : ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
⊢ ({x | f x ≤ B x} ∩ Ioc x y).Nonempty

Tactic:
have HB : ∀ᶠ z in 𝓝[>] x, r < slope B x z :=
  (hasDerivWithinAt_iff_tendsto_slope' <| lt_irrefl x).1 (hB' x xab).Ioi_of_Ici
    (Ioi_mem_nhds hrB)

Result:
case intro.inr.intro.intro
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
s : Set ℝ := {x | f x ≤ B x} ∩ Icc a b
A : ContinuousOn (fun x => (f x, B x)) (Icc a b)
this : IsClosed s
x : ℝ
hxB✝ : f x ≤ B x
xab : x ∈ Ico a b
y : ℝ
hy : y ∈ Ioi x
hxB : f x = B x
r : ℝ
hfr : f' x < r
hrB : r < B' x
hf' : ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
HB : ∀ᶠ (z : ℝ) in 𝓝[>] x, r < slope B x z
⊢ ({x | f x ≤ B x} ∩ Ioc x y).Nonempty

Proof state:
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
hf' : ∀ x ∈ Ico a b, ∀ (r : ℝ), f' x < r → ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
⊢ ∀ ⦃x : ℝ⦄, x ∈ Icc a b → f x ≤ B x

Tactic:
change Icc a b ⊆ { x | f x ≤ B x }

Result:
f f' : ℝ → ℝ
a b : ℝ
hf : ContinuousOn f (Icc a b)
hf' : ∀ x ∈ Ico a b, ∀ (r : ℝ), f' x < r → ∃ᶠ (z : ℝ) in 𝓝[>] x, slope f x z < r
B B' : ℝ → ℝ
ha : f a ≤ B a
hB : ContinuousOn B (Icc a b)
hB' : ∀ x ∈ Ico a b, HasDerivWithinAt B (B' x) (Ici x) x
bound : ∀ x ∈ Ico a b, f x = B x → f' x < B' x
⊢ Icc a b ⊆ {x | f x ≤ B x}