`D = ( x - 1 )( x + 2 )( x + 3 )( x + 6 ) + 2026`

`D = [ ( x - 1 )( x + 6 ) ][ ( x + 2 )( x + 3 ) ] + 2026`

`D = ( x^2 + 5x - 6 )( x^2 + 5x + 6 ) + 2026`

`D = ( x^2 + 5x )^2 - 36 + 2026`

`D = ( x^2 + 5x )^2 + 1990`

`Do  ( x^2 + 5x )^2 >= 0  AA  x`

`D = ( x^2 + 5x )^2 + 1990 >= 1990  AA  x`

`Dấu = xảy  ra:  ( x^2 + 5x )^2=  0`

`x( x + 5 ) = 0`

`x = 0  hoặc   x =- 5`

`Vậy  D min = 1990` khi `x in { 0 ; -5 }`