Đáp án:
`A = 3/10`
Giải thích các bước giải:
`A = 2/2.5 + 2/5.8 + 2/8.11 + 2/11.14 + 2/14.17 + 2/17.20`
`= 2/3 . ( 3/2.5 + 3/5.8 + 3/8.11 + 3/11.14 + 3/14.17 + 3/17.20 )`
`= 2/3 . ( 1/2-1/5+1/5-1/8+1/8-1/11+1/11-1/14+1/14-1/17+1/17-1/20 )`
`= 2/3 . ( 1/2-1/20 ) = 2/3 . 9/20 = 3/10`
Vậy `A = 3/10`