Tìm x biết:

a) \({x^3} - 6{x^2} + 12x + 56 = 0\)

\({x^3} - 6{x^2} + 12x - 8 + 64 = 0\)

\({\left( {x - 2} \right)^3} =  - 64\)

Suy ra \(x - 2 =  - 4\)

\(x =  - 2.\)

Vậy \(x =  - 2.\)

b) \[3{\left( {x + 2} \right)^2} + {\left( {2x - 1} \right)^2} - 7\left( {x + 3} \right)\left( {x - 3} \right) = 36\]

\[3\left( {{x^2} + 4x + 4} \right) + \left( {4{x^2} - 4x + 1} \right) - 7\left( {{x^2} - 9} \right) = 36\]

\[3{x^2} + 12x + 12 + 4{x^2} - 4x + 1 - 7{x^2} + 63 = 36\]

\[\left( {3{x^2} + 4{x^2} - 7{x^2}} \right) + \left( {12x - 4x} \right) + \left( {12 + 1 + 63} \right) = 36\]

\[8x =  - 40\]

\[x =  - 5.\]

Vậy \[x =  - 5.\]