a/ `x^3 + 12 = 3x^2 + 4x`
`\Leftrightarrow x^3 -3x^2 - 4x + 12 = 0`
`\Leftrightarrow x^2( x -3 ) - 4( x - 3 ) = 0`
`\Leftrightarrow ( x - 3 )( x -2 )( x + 2 ) = 0`
`\Leftrightarrow` \(\left[ \begin{array}{l}x=3\\x=2\\x=-2\end{array} \right.\)
`S =` { `3;2;-2` }
b/ `x^4 +2x^3 + 5x^2 + 4x - 12 = 0`
`\Leftrightarrow x^4 - x^3 + 3x^3 - 3x^2 + 8x^2 - 8x + 12x - 12 = 0`
`\Leftrightarrow x^3( x -1 ) + 3x^2( x - 1 ) + 8x( x - 1) + 12 ( x - 1 ) = 0`
`\Leftrightarrow ( x - 1)( x^3 + 3x^2 +8x + 12 ) = 0`
`\Leftrightarrow ( x - 1 )( x^3 + 2x^2 + x^2 + 2x + 6x + 12 ) = 0`
`\Leftrightarrow ( x - 1)[ x^2( x + 2 ) + x( x + 2 ) + 6( x + 2 ) ]= 0`
`\Leftrightarrow ( x - 1 )( x + 2)( x^2 + x + 6 ) = 0`
`\Leftrightarrow` \(\left[ \begin{array}{l}x-1=0\\x+2=0\\x^2 + x + 6 = 0 ( vn )\end{array} \right.\)
`\Leftrightarrow` \(\left[ \begin{array}{l}x=1\\x=-2\\\end{array} \right.\)
`S =` { `1;-2` }
