( x + 3 )^3 = 9 ( x + 3 )
<=> x^3 + 9x^2 + 27x + 27 = 9x + 27
<=> x^3 + 9x^2 +18x = 0
<=> x( x^2 + 9x + 18 ) =0
<=> x( x + 3)( x + 6 ) =0
<=> x = 0 ; -3 ; -6 (*)
( x + 3 )^3 - 9( x + 3) = 0
<=> ( x + 3 )[ ( x + 3 )^2 - 9 ) = 0
<=> ( x + 3 )( x + 3 - 3 )( x + 3 + 3) =0
<=> ( x + 3 )x(x + 6 )=0
<=> x = -3 ; 0 ; -6 (**)
Từ (*) ; (**) Có là cặp tương ứng.