Đáp án`+`Giải thích các bước giải:

`f)`

`5/13+(-5)/7+(-20)/41+8/13+(-21)/41`

`=(5/13+8/13)+((-20)/41+(-21)/41)+(-5)/7`

`=1+(-1)+(-5)/7`

`=0+(-5)/7=(-5)/7`

`g)`

`1/15+1/35+1/63+1/99+1/143`

`=1/(3.5)+1/(5.7)+1/(7.9)+1/(9.11)+1/(11.13)`

`=1/2.(2/(3.5)+2/(5.7)+2/(7.9)+2/(9.11)+2/(11.13))` 

`=1/2.(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13)`

`=1/2(1/3-1/13)`

`=1/2 . 10/39`

`=5/39`

`3.`

`a) x=(-1)/4+2/13

`x=(-5)/52`

Vậy `x=(-5)/52`

`b)`

`x/3=2/3+(-1)/7`

`x/3=11/21`

`(7x)/21=11/21`

`=> 7x=11`

`=>x=11:7`

`=>x=11/7`

Vậy `x=11/7`