Đặt `S= 1+2+3+4+5+6+7+8+9+10+11+12+...`

`S= 1+(2+3+4)+(5+6+7)+(8+9+10)+...`

`S= 1+9+18+27+...`

`S= 1+9.(1+2+3+...)`

`=>S= 1+9S`

`=> S-9S=1`

`=>-8S=1`

`=>S= -1/8`

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t lm v đko v

ae jup t vs T_T