S= 7/4x(3333/1212+3333/2020+3333/3030+3333/4242)

`S=7/4×(3333/1212+3333/2020+3333/3030+3333/4242)`

`S=7/4×(11/4+33/20+11/10+11/14)`

`S=7/4×(55/20+33/20+11/10+11/14)`

`S=7/4×(22/5+11/10+11/14)`

`S=7/4×(44/10+11/10+11/14)`

`S=7/4×(11/2+11/14)`

`S=7/4×(77/14+11/14)`

`S=7/4×44/7`

`S=11`

Vậy `S=11`

`Li`nh `Nguyễn`