`Vì 1/5 +1/6 > 1/17 . 6 = 6/17`
`⇒ 1/7 + 1/8 + 1/9 + ......... + 1/17 > 1/17 + 1/17 + 1/17 + .......... + 1/17 = 1/17 . 11 = 11/17`
`⇒ 1/5 + 1/6 + 1/7 + ......... + 1/17 > 6/17 + 11/17 = 17/17 = 1`
`⇒ 1 < 1/5 + 1/6 + 1/7 + ......... + 1/17`
`text(Ta có :)`
`1/5 + 1/6 +1 /7 + ......... + 1/17 = (1/5 + 1/6 + ........ + 1/10) + (1/11 + 1/12 +........ + 1/17)`
`< (1/10 + 1/10 + ......... + 1/10) + (1/17 + 1/17 + ........ + 1/17)`
`=1/10 . 6 + 1/17 . 7 = 86/85 < 2`
`Vậy 1 < 1/5 + 1/6 + 1/7 + ......... + 1/17 < 2`