no goals
α : Type u
β : Type v
γ : Type w
s₂ s₁ s : Finset α
a : α
g f : α → β
inst✝ : CommMonoid β
n : ℕ
⊢ ∑ i in range n, i = n * (n - 1) / 2
rw [← sum_range_id_mul_two n, Nat.mul_div_cancel _ zero_lt_two]