1乘1+2乘2+3乘3+4乘4+5乘5+6乘6+7乘7+……+1991乘1991=1991×（1991+1）×（1991×2+1）÷6=1991×1992×3983÷6=1991×332×39831×2×3=61乘1+2乘2+3乘3+4乘4+5乘5+6乘6+7乘7+……+1991乘1991的末位数字是6
nx(n+1)x(2n+1)/6 把1991带进去得2632810796 末位是6
6
