1x2+2x3+3x4+4x5+5x6+.+99x100

来源：学生作业帮编辑：灵鹊做题网作业帮分类：数学作业时间：2024/04/27 02:22:11

∑(k-1)k=∑k^2 - ∑k=(n-1)n(n+1)/3,(k=1,2,3...n)
把100代入上式
1x2+2x3+3x4+4x5+5x6+...+99x100=99*100*101/3= 333300
再问：没看明白
再答： ∑k^2 =n（n+1）(2n+1)/6 ∑k=n+n^2/2+n/2 然后1式减2式，得到（n-1）n(n+1)/3 会数列求和吗？

1x1-2x2+3x3-4x4+5x5-6x6.-100x100+101x101 计算1x2+2x3+3x4+4x5+...+99x100 1/1x2+1/2x3+1/3x4+1/4x5+.+1/98x99+1/99x100 1/1X2+1/2x3+1/3x4+1/4x5+.+1/98x99+1/99x100求结果! 2/1x2+2/2x3+2/3x4+2/4x5+...+2/98x99+2/99x100 一定要帮我啊!(1/1x2)+(1/2x3)+(1/3x4)+(1/4x5)+.+(1/99x100)=? 求1x2,2x3,3x4,4x5,……98x99,99x100的倒数和 1/1X2+1/2X3+1/3X4+1/4X5+1/5X6……1/2007X2008 1x2/1+2x3/1+3x4/1+4x5/1+5x6/1+6x7/1 1/1x2+1/2x3+1/3x4+1/4x5+1/5x6+......1/2008x2009 1/1x2+1/2x3+1/3x4+1/4x5+1/5x6+1/6x7 (2X2)/1X3+(4X4)/(3X5)+(6X6)/(5X7)+...+(20X20)/(19X21)==?