s=1.2+3.4+...+99.100

`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.(3-0) + 2.3.(4-1) + 3.4.(5-2) + ... + 99.100 . (101 - 98)`

`=> 3S = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 99.100.101 - 98.99.100`

`=> 3S = 0.1.2 + (1.2.3 - 1.2.3) + (2.3.4 - 2.3.4) + ... + (98.99.100 - 98.99 . 100) + 99.100.101`

`=> 3S = 0 + 0 + 0 + ... + 0 + 99.100.101`

`=> 3S = 99.100.101`

`=> 3S = 999900`

`=> S = 999900 : 3`

`=> S = 333300`

Vậy `S = 333300`

`#dtkc`