Đáp án:
`A=5/1+5/3+5/9+5/27+5/81+5/243`
`A=5(1+1/3+1/9+1/27+1/81+1/243)`
Đặt `B=1+1/3+1/9+1/27+1/81+1/243`
`B=1+1/3+1/(3xx3)+1/(9xx3)+1/(27xx3)+1/(81xx3)`
`3B=3(1+1/3+1/(3xx3)+1/(9xx3)+1/(27xx3)+1/(81xx3))`
`3B=3+1+1/3+1/9+1/27+1/81`
`2B=3B-B=3-1/243`
`2B=728/243`
`B=728/243:2`
`B=364/243`
Từ đó tìm được.
`A=5xxB`
`A=5xx364/243`
`A=1820/243`
Vậy `A=1820/243`