`1/3 -1/12 - 1/20-1/30 -1/42-1/56 - 1/72 -1/90-1/110=x-5/13`

`1/3 - (1/12 +1/20+1/30+1/42+1/56+1/72+1/90+1/110) = x - 5/13`

Đặt `A = 1/12 +1/20+1/30+1/42+1/56+1/72+1/90+1/110`

`A = 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 +1/9.10 + 1/10.11`

`A= 1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10 +1/10-1/11`

`A = 1/3-1/11`

`A = 8/33`

Thay `A` vào, ta có :

`1/3-8/33 = x-5/13`

`11/33-8/33 = x-5/13`

`3/33 =x-5/13`

`1/11 = x-5/13`

`x -5/13 =1/11`

`x = 1/11+5/13`

`x=  68/143`

Vậy `x=68/143`