-- Mathlib/Analysis/Calculus/Darboux.lean
theorem exists_hasDerivWithinAt_eq_of_gt_of_lt (hab : a ≤ b)
    (hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x) {m : ℝ} (hma : f' a < m)
    (hmb : m < f' b) : m ∈ f' '' Ioo a b

Proof state:
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
x : ℝ
hx : x ∈ Icc a b
⊢ HasDerivWithinAt g (f' x - m) (Icc a b) x

Tactic:
simpa using (hf x hx).sub ((hasDerivWithinAt_id x _).const_mul m)

Result:
no goals

Proof state:
case inr.inl
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
cmem : b ∈ Icc a b
hc : IsMinOn g (Icc a b) b
hac : a < b
⊢ 0 ≤ (a - b) * (f' b - m)

Tactic:
have : a - b ∈ posTangentConeAt (Icc a b) b :=
  sub_mem_posTangentConeAt_of_segment_subset (by rw [segment_symm, segment_eq_Icc hab])

Result:
case inr.inl
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
cmem : b ∈ Icc a b
hc : IsMinOn g (Icc a b) b
hac : a < b
this : a - b ∈ posTangentConeAt (Icc a b) b
⊢ 0 ≤ (a - b) * (f' b - m)

Proof state:
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
cmem : b ∈ Icc a b
hc : IsMinOn g (Icc a b) b
hac : a < b
⊢ segment ℝ b a ⊆ Icc a b

Tactic:
rw [segment_symm, segment_eq_Icc hab]

Result:
no goals

Proof state:
case inr.inl
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
cmem : b ∈ Icc a b
hc : IsMinOn g (Icc a b) b
hac : a < b
⊢ b ∈ Ioo a b

Tactic:
refine absurd (sub_nonpos.1 <| nonpos_of_mul_nonneg_right ?_ (sub_lt_zero.2 hab'))
  (not_le_of_lt hmb)

Result:
case inr.inl
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
cmem : b ∈ Icc a b
hc : IsMinOn g (Icc a b) b
hac : a < b
⊢ 0 ≤ (a - b) * (f' b - m)

Proof state:
case inl
a : ℝ
f f' : ℝ → ℝ
m : ℝ
hma : f' a < m
hab : a ≤ a
hf : ∀ x ∈ Icc a a, HasDerivWithinAt f (f' x) (Icc a a) x
hmb : m < f' a
⊢ m ∈ f' '' Ioo a a

Tactic:
exact (lt_asymm hma hmb).elim

Result:
no goals

Proof state:
case inr.inl
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
cmem : b ∈ Icc a b
hc : IsMinOn g (Icc a b) b
hac : a < b
this : a - b ∈ posTangentConeAt (Icc a b) b
⊢ 0 ≤ (a - b) * (f' b - m)

Tactic:
simpa only [ContinuousLinearMap.smulRight_apply, ContinuousLinearMap.one_apply]
  using hc.localize.hasFDerivWithinAt_nonneg (hg b (right_mem_Icc.2 hab)) this

Result:
no goals

Proof state:
case inl
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
cmem : a ∈ Icc a b
hc : IsMinOn g (Icc a b) a
this : b - a ∈ posTangentConeAt (Icc a b) a
⊢ 0 ≤ (b - a) * (f' a - m)

Tactic:
simpa only [ContinuousLinearMap.smulRight_apply, ContinuousLinearMap.one_apply]
  using hc.localize.hasFDerivWithinAt_nonneg (hg a (left_mem_Icc.2 hab)) this

Result:
no goals

Proof state:
case right
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
c : ℝ
cmem : c ∈ Icc a b
hc : IsMinOn g (Icc a b) c
cmem' : c ∈ Ioo a b
⊢ f' c - m = 0

Tactic:
have : Icc a b ∈ 𝓝 c := by rwa [← mem_interior_iff_mem_nhds, interior_Icc]

Result:
case right
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
c : ℝ
cmem : c ∈ Icc a b
hc : IsMinOn g (Icc a b) c
cmem' : c ∈ Ioo a b
this : Icc a b ∈ 𝓝 c
⊢ f' c - m = 0

Proof state:
case right
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
c : ℝ
cmem : c ∈ Icc a b
hc : IsMinOn g (Icc a b) c
cmem' : c ∈ Ioo a b
this : Icc a b ∈ 𝓝 c
⊢ f' c - m = 0

Tactic:
exact (hc.isLocalMin this).hasDerivAt_eq_zero ((hg c cmem).hasDerivAt this)

Result:
no goals

Proof state:
case inl
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
cmem : a ∈ Icc a b
hc : IsMinOn g (Icc a b) a
⊢ 0 ≤ (b - a) * (f' a - m)

Tactic:
have : b - a ∈ posTangentConeAt (Icc a b) a :=
  sub_mem_posTangentConeAt_of_segment_subset (segment_eq_Icc hab ▸ Subset.rfl)

Result:
case inl
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
hg : ∀ x ∈ Icc a b, HasDerivWithinAt g (f' x - m) (Icc a b) x
cmem : a ∈ Icc a b
hc : IsMinOn g (Icc a b) a
this : b - a ∈ posTangentConeAt (Icc a b) a
⊢ 0 ≤ (b - a) * (f' a - m)

Proof state:
case inr
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
⊢ m ∈ f' '' Ioo a b

Tactic:
set g : ℝ → ℝ := fun x => f x - m * x

Result:
case inr
a b : ℝ
f f' : ℝ → ℝ
hab : a ≤ b
hf : ∀ x ∈ Icc a b, HasDerivWithinAt f (f' x) (Icc a b) x
m : ℝ
hma : f' a < m
hmb : m < f' b
hab' : a < b
g : ℝ → ℝ := fun x => f x - m * x
⊢ m ∈ f' '' Ioo a b