-- Mathlib/RingTheory/Polynomial/Bernstein.lean
theorem variance (n : ℕ) :
    (∑ ν ∈ Finset.range (n + 1), (n • Polynomial.X - (ν : R[X])) ^ 2 * bernsteinPolynomial R n ν) =
      n • Polynomial.X * ((1 : R[X]) - Polynomial.X)

Proof state:
R : Type u_1
inst✝ : CommRing R
n : ℕ
⊢ ∑ ν ∈ Finset.range (n + 1), (n • X - ↑ν) ^ 2 * bernsteinPolynomial R n ν = n • X * (1 - X)

Tactic:
have p : ((((Finset.range (n + 1)).sum fun ν => (ν * (ν - 1)) • bernsteinPolynomial R n ν) +
    (1 - (2 * n) • Polynomial.X) * (Finset.range (n + 1)).sum fun ν =>
      ν • bernsteinPolynomial R n ν) + n ^ 2 • X ^ 2 *
        (Finset.range (n + 1)).sum fun ν => bernsteinPolynomial R n ν) = _ :=
  rfl

Result:
R : Type u_1
inst✝ : CommRing R
n : ℕ
p :
  ∑ ν ∈ Finset.range (n + 1), (ν * (ν - 1)) • bernsteinPolynomial R n ν +
        (1 - (2 * n) • X) * ∑ ν ∈ Finset.range (n + 1), ν • bernsteinPolynomial R n ν +
      n ^ 2 • X ^ 2 * ∑ ν ∈ Finset.range (n + 1), bernsteinPolynomial R n ν =
    ∑ ν ∈ Finset.range (n + 1), (ν * (ν - 1)) • bernsteinPolynomial R n ν +
        (1 - (2 * n) • X) * ∑ ν ∈ Finset.range (n + 1), ν • bernsteinPolynomial R n ν +
      n ^ 2 • X ^ 2 * ∑ ν ∈ Finset.range (n + 1), bernsteinPolynomial R n ν
⊢ ∑ ν ∈ Finset.range (n + 1), (n • X - ↑ν) ^ 2 * bernsteinPolynomial R n ν = n • X * (1 - X)

Proof state:
case calc_2
R : Type u_1
inst✝ : CommRing R
n : ℕ
p :
  ∑ x ∈ Finset.range (n + 1),
      (↑(x * (x - 1)) + (1 - ↑(2 * n) * X) * ↑x + ↑(n ^ 2) * X ^ 2) * bernsteinPolynomial R n x =
    ↑(n * (n - 1)) * X ^ 2 + (1 - (2 * n) • X) * n • X + n ^ 2 • X ^ 2 * 1
⊢ ↑n * ↑(n - 1) * X ^ 2 + (1 - 2 * ↑n * X) * (↑n * X) + ↑n ^ 2 * X ^ 2 * 1 = ↑n * X * (1 - X)

Tactic:
cases n

Result:
case calc_2.zero
R : Type u_1
inst✝ : CommRing R
p :
  ∑ x ∈ Finset.range (0 + 1),
      (↑(x * (x - 1)) + (1 - ↑(2 * 0) * X) * ↑x + ↑(0 ^ 2) * X ^ 2) * bernsteinPolynomial R 0 x =
    ↑(0 * (0 - 1)) * X ^ 2 + (1 - (2 * 0) • X) * 0 • X + 0 ^ 2 • X ^ 2 * 1
⊢ ↑0 * ↑(0 - 1) * X ^ 2 + (1 - 2 * ↑0 * X) * (↑0 * X) + ↑0 ^ 2 * X ^ 2 * 1 = ↑0 * X * (1 - X)

case calc_2.succ
R : Type u_1
inst✝ : CommRing R
n✝ : ℕ
p :
  ∑ x ∈ Finset.range (n✝ + 1 + 1),
      (↑(x * (x - 1)) + (1 - ↑(2 * (n✝ + 1)) * X) * ↑x + ↑((n✝ + 1) ^ 2) * X ^ 2) * bernsteinPolynomial R (n✝ + 1) x =
    ↑((n✝ + 1) * (n✝ + 1 - 1)) * X ^ 2 + (1 - (2 * (n✝ + 1)) • X) * (n✝ + 1) • X + (n✝ + 1) ^ 2 • X ^ 2 * 1
⊢ ↑(n✝ + 1) * ↑(n✝ + 1 - 1) * X ^ 2 + (1 - 2 * ↑(n✝ + 1) * X) * (↑(n✝ + 1) * X) + ↑(n✝ + 1) ^ 2 * X ^ 2 * 1 =
    ↑(n✝ + 1) * X * (1 - X)

Proof state:
R : Type u_1
inst✝ : CommRing R
n : ℕ
p :
  ∑ ν ∈ Finset.range (n + 1), (ν * (ν - 1)) • bernsteinPolynomial R n ν +
        (1 - (2 * n) • X) * ∑ ν ∈ Finset.range (n + 1), ν • bernsteinPolynomial R n ν +
      n ^ 2 • X ^ 2 * ∑ ν ∈ Finset.range (n + 1), bernsteinPolynomial R n ν =
    ∑ ν ∈ Finset.range (n + 1), (ν * (ν - 1)) • bernsteinPolynomial R n ν +
        (1 - (2 * n) • X) * ∑ ν ∈ Finset.range (n + 1), ν • bernsteinPolynomial R n ν +
      n ^ 2 • X ^ 2 * ∑ ν ∈ Finset.range (n + 1), bernsteinPolynomial R n ν
⊢ ∑ ν ∈ Finset.range (n + 1), (n • X - ↑ν) ^ 2 * bernsteinPolynomial R n ν = n • X * (1 - X)

Tactic:
conv at p =>
  lhs
  rw [Finset.mul_sum, Finset.mul_sum, ← Finset.sum_add_distrib, ← Finset.sum_add_distrib]
  simp only [← natCast_mul]
  simp only [← mul_assoc]
  simp only [← add_mul]

Result:
R : Type u_1
inst✝ : CommRing R
n : ℕ
p :
  ∑ x ∈ Finset.range (n + 1),
      (↑(x * (x - 1)) + (1 - ↑(2 * n) * X) * ↑x + ↑(n ^ 2) * X ^ 2) * bernsteinPolynomial R n x =
    ∑ ν ∈ Finset.range (n + 1), (ν * (ν - 1)) • bernsteinPolynomial R n ν +
        (1 - (2 * n) • X) * ∑ ν ∈ Finset.range (n + 1), ν • bernsteinPolynomial R n ν +
      n ^ 2 • X ^ 2 * ∑ ν ∈ Finset.range (n + 1), bernsteinPolynomial R n ν
⊢ ∑ ν ∈ Finset.range (n + 1), (n • X - ↑ν) ^ 2 * bernsteinPolynomial R n ν = n • X * (1 - X)