`B = 1/3 - 1/3^2 - 1/3^3 -...-1/3^100`
`1/3 B= 1/3( 1/3 - 1/3^2 - 1/3^3 -...-1/3^100)`
`1/3 B= 1/3^2 - 1/3^3 - 1/3^4-...-1/3^101`
`B- 1/3 B= 1/3 - 1/3^2 - 1/3^3 -...- 1/3^100 - 1/3^2 + 1/3^3 + 1/3^4 +...+1/3^101`
`2/3 B= 1/3 - 1/3^2 - 1/3^2 + 1/3^101`
`2/3 B= 1/3^2 + 1/3^101`
`B= (1/3^2 + 1/3^101) : 2/3`
`B= (1/3^2 + 1/3^101) . 3/2`
`B= 1/6 + 1/(3^100 .2)`
Vậy `B= 1/6 + 1/(3^100 .2)`
