1x2+2x3+3x4+4x5+5x6+6x7+7x8+8x9=

分类: 作业答案 • 2021-12-20 10:47:39

问题描述：

答

1x2 + 2x3 + 3x4 + .+ n(n+1) = 1/3 * n(n+1)(n+2)
1x2+2x3+3x4+4x5+5x6+6x7+7x8+8x9
= 1/3 * 8 * 9 * 10
= 240