Question

求一道初中数学题的简便算法1×2＋2×3＋3×4＋4×5.(2005＋2005×2006)

Answer 1

1×2+2×3+3×4+4×5+……+99×100 +...n(n+1)

=n(n+1)(n+2)/3;

原式=（2005×2006×2007)/3＝2690738070