Đáp án:
`S = 1 . 2 + 2 . 3 + 3 . 4 + .................. + 99 . 100`
`3S = 1 . 2 . 3 + 2 . 3 . 3 + 3 . 4 . 3 + .......... + 99 . 100 . 3`
`3S = 1 . 2 . 3 + 2 . 3 . ( 4 - 1) + 3 . 4 . ( 5 - 2) + .......... + 99 . 100 . (101 - 98)`
` = 1 . 2 . 3 - 1 . 2 . 3 + 2 . 3 . 4 - 2 . 3 . 4 + ......... - 98 . 99 . 100 + 99 . 100 . 101`
` = 99 . 100 . 101`
` S =(99 . 100 . 101)/3`
` = 33 . 100 . 101 = 3300 . 101 = 333300`
`text{ Mathvietnam2k8}`
