qify [htwodvd, bigger]
n✝ : ℕ
ih : (∑ x in Finset.range (n✝ + 1), x) ^ 2 = ∑ x in Finset.range (n✝ + 1), x ^ 3
S : ℕ := ∑ x in Finset.range (n✝ + 1), x
hS : S = ∑ x in Finset.range (n✝ + 1), x
bigger : ∀ (n : ℕ), 1 ≤ n + 1
n : ℤ
htwodvd : 2 ∣ (n + 1) * (n + 1 - 1)
⊢ (n + 1) ^ 2 + 2 * (n + 1) * ((n + 1) * (n + 1 - 1) / 2) = (n + 1) ^ 3
n✝ : ℕ
ih : (∑ x in Finset.range (n✝ + 1), x) ^ 2 = ∑ x in Finset.range (n✝ + 1), x ^ 3
S : ℕ := ∑ x in Finset.range (n✝ + 1), x
hS : S = ∑ x in Finset.range (n✝ + 1), x
bigger : ∀ (n : ℕ), 1 ≤ n + 1
n : ℤ
htwodvd : 2 ∣ (n + 1) * (n + 1 - 1)
⊢ (↑n + 1) ^ 2 + 2 * (↑n + 1) * ((↑n + 1) * (↑n + 1 - 1) / 2) = (↑n + 1) ^ 3