Clever calculation: 1x2 + 2x3 + 3x4 + 4x5 + 5x6 + 6x7... + 8x9

Clever calculation: 1x2 + 2x3 + 3x4 + 4x5 + 5x6 + 6x7... + 8x9


1 × 2 + 2 × 3 +... + 8 × 9 is too long to be copied completely. = 1 × (1 + 1) + 2 × (2 + 1) +... + 8 × (8 + 1) = 1 & # 178; + 2 & # 178; +... + 8 & # 178; + 1 + 2 +... + 8 = 8 × 9 × 17 / 6 + 8 × 9 / 2 = 204 + 36 = 240 use the formula: 1 + 2 +... + n = n (n + 1) / 21 & # 178; + 2 & # 178; +... + n & # 178; = n (n + 1) (2