giúp mình bài này vs ạ mình đg cần gấp ạ

Answer.

Bài `6:`

`S=3/1.6+3/6.11+3/11.16+...+3/2011.2016+3/2016.2021`

`=>S=3.(1/1.6+1/6.11+1/11.16+...+1/2011.2016+1/2016.2021)`

`=>5S=3.(5/1.6+5/6.11+5/11.16+...+5/2011.2016+5/2016.2021)`

`=>5S=3.(1/1-1/6+1/6-1/11+1/11-1/16+...+1/2011-1/2016+1/2016-1/2021)`

`=>5S=3.(1/1-1/2021)`

`=>5S=3. 2020/2021`

`=>S=1212/2021`

Ta có: `1212/2021=2424/4042`

`1/2=2021/4042`

Vì `2424>2021`

`=>2424/4042>2021/4042`

`=>S>1/2`

Vậy `S>1/2`