`a) 3/7 - 1/21 x = 1/3`
`1/21 x = 3/7 - 1/3`
`1/21 x = 2/21`
`x = 2/21 : 1/21`
`x= 2`
Vậy `x = 2`
``
`b) 7/6 - x : 3/4 = 1/12`
`x : 3/4 = 7/6 - 1/12`
`x : 3/4 = 13/12`
`x = 13/12 . 3/4`
`x= 13/16`
Vậy `x = 13/16`
``
`c) ( x - 2/7) ( x + 3/4) = 0`
⇒ `+)` Trường hợp `1` :
`x - 2/7 = 0`
`x =0 + 2/7`
`x = 2/7`
`+)` Trường hợp `2` :
`x + 3/4 = 0`
`x = 0 - 3/4`
`x = -3/4`
Vậy `x ∈ { 2/7 ; -3/4}`
``
`d) ( -5/4 x + 3,25) [3/5 - (-5/2x)] = 0`
`(-5/4 x + 13/4) ( 3/5 + 5/2x) = 0`
⇒ `+)` Trường hợp `1` :
`-5/4 x + 13/4 = 0`
`-5/4 x = 0 - 13/4`
`-5/4 x = -13/4`
`x = -13/4 : (-5/4)`
`x = 13/5`
`+)` Trường hợp `2` :
`3/5 + 5/2x = 0`
`5/2 x = 0 - 3/5`
`5/2 x = -3/5`
`x = -3/5 : 5/2`
`x= -6/25`
Vậy `x ∈ { 13/5 ; -6/25}`
