`text{Đáp án:}`
`A < 1`
`text{Giải thích các bước giải:}`
`A = 2/1.3 - 2/2.4 + 2/3.5 - 2/4.6 + 2/5.7 - 2/6.8 + 2/7.9 - 2/8.10 + 2/9.11 - 2/10.12 `
`A = 2/3 - 2/8 + 2/15 - 2/24 + 2/35 - 2/48 + 2/63 - 2/80 + 2/99 - 2/120`
`A = 2/3 - 1/4 + 2/15 - 1/12 + 2/35 - 1/24 + 2/63 - 1/40 + 2/99 - 1/60`
`A = (2/3 - 1/4) + (2/15 - 1/12) + (2/35 - 1/24) + (2/63 - 1/40) + (2/99 - 1/60)`
`A = 5/12 + 1/20 + 13/840 + 17/2520 + 7/1980`
`A = (5/12 + 1/20) + (13/840 + 17/2520 + 7/1980)`
`A = 7/15 + 17/660`
`A = 65/132`
`→` `65/132 < 1`
`⇒` `A < 1`