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`3)`

`( x - 1/3 )^3 = (-8)/27`

`( x - 1/3 )^3 = ( (-2)/3 )^3`

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

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

`x = (-1)/3`

Vậy `x = (-1)/3`

`6)`

`( x - 1/8 )^3 = (-8)/125`

`( x - 1/8 )^3 = ( (-2)/5 )^3`

`x - 1/8 = (-2)/5`

`x = (-2)/5 + 1/8`

`x = (-11)/40`

Vậy `x = (-11)/40`

`9)`

`( x - 5/6 )^2 = 1/36`

`( x - 5/6 )^2 = ( ±1/6 )^2`

`TH1 : x - 5/6 = 1/6`

         `x = 1/6 + 5/6`

         `x = 1`

`TH2 : x - 5/6 = -1/6`

         `x = -1/6 + 5/6`

         `x = 2/3`

Vậy `x = { 1 ; 2/3 }`

`12)`

`( x - 4/7 )^2 = 1/49`

`( x - 4/7 )^2 = ( ±1/7 )^2`

`TH1 : x - 4/7 = 1/7`

          `x = 1/7 + 4/7`

          `x = 5/7`

`TH2 : x - 4/7 = -1/7`

          `x = -1/7 + 4/7`

          `x = 3/7`

Vậy `x = { 5/7 ; 3/7 }`