`x - 1/3 = 6/7`

`x = 6/7 + 1/3`

`x = 18/21 + 7/21`

`x = 25/21`

Vậy `x = 25/21`

`-----------`

`12/7 - x = 1/3`

`x = 12/7 - 1/3`

`x = 36/21 - 7/21`

`x = 29/21`

Vậy `x = 29/21`

`-----------`

`5/9 xx x = 2/3`

`x = 2/3 : 5/9`

`x = 2/3 xx 9/5`

`x = 6/5`

Vậy `x = 6/5`

`-----------`

`12/13 : 9/26`

`= 12/13 xx 26/9`

`= (12 xx 26)/(13 xx 9)`

`= (3 xx 4 xx 13 xx 2)/(13 xx 3 xx 3)`

`= (4 xx 2)/3`

`= 8/3`

`-----------`

`3/5 xx 15/7`

`= (3 xx 15)/(5 xx 7)`

`= (3 xx 3 xx 5)/(5 xx 7)`

`= (3 xx 3)/7`

`= 9/7`

`-----------`

`x + 5/7 = 4/5`

`x = 4/5 - 5/7`

`x = 28/35 - 25/35`

`x = 3/35`

Vậy `x = 3/35`

`\ttcolor{cyan}{#PUyen}`