A series of numbers in regular order: 1, 3, 7, 13, 21, 31 What's the number in 2008? It's better to have a formula

1＝0×1＋1,3＝1×2＋1,7＝2×3+1,…… The n is n × (n-1) + 1, so the 2008 is 2008 × 2007 + 1 = 4030057

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