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`a) ( 4x - 1 )( 3 + x ) = ( 12x^3 + 9x^2 - 21x ) : ( 3x )`

`4x^2 + 11x - 3 = [ 3x( 4x^2 + 3x - 7 ) ] : ( 3x )`

`4x^2 + 11x - 3 = 4x^2 + 3x - 7`

`8x = -4`

`x = -1/2`

`Vậy  x = -1/2`

`b) M = ( x - y )^2 - 2x - 2y`

`M = x^2 - 2xy + y^2 - 2( x + y )`

`M = x^2 + 2xy + y^2 - 2( x + y ) - 4xy`

`M = ( x + y )^2 - 2( x + y ) - 4xy`

`M = 5^2 - 2 . 5 - 4 . 6`

`M = 25 - 10 - 24`

`M = -9`