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											2 years ago
+								import Mathlib.Tactic.Ring
 								import Mathlib.Algebra.BigOperators.Fin
 								open BigOperators
 								lemma arithmetic_sum (n : ℕ) : 2 * (∑ i : Fin (n + 1), ↑i) = n * (n + 1) := by
 								  induction' n with n hn
 								  simp
 								  rw [Fin.sum_univ_castSucc]
 								  rw [mul_add]
 								  simp
 								  rw [mul_add, hn]
 								  simp_rw [Nat.succ_eq_one_add]
 								  ring