`A = 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729`
`3 xx A = 3 xx ( 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729 )`
`3 xx A = 3 + 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243`
`3 xx A - A = ( 3 + 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 ) - ( 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729 )`
`2 xx A = 3 - 1/729`
`2 xx A = 2186/729`
`A = 2186/729 : 2`
`A = 1093/729`
Vậy `, A = 1093/729 .`
