S = 1 x 200 + 2 x 199 + 3 x 198 + 4 x 197 + ....... + 199 x 2 + 200 x 1 Giúp em với ạ. Đang cần gấp, ai trả lời nhanh nhất em cho cảm ơn ạ

`S = 1 xx 200 + 2 xx 199 + 3 xx 198 + 4 xx 197 + ... + 199 xx 2 + 200 xx 1`

`S = 200 xx 200 - 199 xx 200 + 200 xx 199 - 198 xx 199 + ... + 200 xx 2 - 1 xx 2 + 200 xx 1`

`S = (200 xx 200 + 200 xx 199 + ... + 200 xx 2 + 200 xx 1) - (1 xx 2 + 2 xx 3 + 3 xx 4 + ... + 199 xx 200)`

`S = 200(200 + 199 + ... + 2 + 1) - [1 xx 2 xx 3 + 2 xx 3 xx (4 - 1) + 3 xx 4 xx (5 - 2) + ... + 199 xx 200 xx (201- 198)] : 3`

`S = 200 xx 20100 - [1 xx 2 xx 3 - 1 xx 2 xx 3 + 2 xx 3 xx 4 - 2 xx 3 xx 4 + 3 xx 4 xx 5 - ... - 198 xx 199 xx 200 + 199 xx 200 xx 201] : 3`

`S = 4020000 - [199 xx 200 xx 201] : 3`

`S = 4020000 - 7999800 : 3`

`S = 4020000 - 2666600`

`S = 1353400`

       Vậy: `S = 1353400`

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