x-2y `vdots` 3
$\Rightarrow$ $(x-2y)^{3}$ `vdots` 9
$\Rightarrow$ $x^{3}$ - $6x^{2}$y + $12xy^{2}$ - $8y^{3}$ `vdots` 9
Ta có:
- $6x^{2}$y + $12xy^{2}$=-6(x-2y)
Vì -6 `vdots` 3 và x-2y `vdots` 3
$\Rightarrow$ -6(x-2y) `vdots` 3x3
$\Rightarrow$ -6(x-2y) `vdots` 9
$\Rightarrow$- $6x^{2}$y + $12xy^{2}$ `vdots`3
$\Rightarrow$ $x^{3}$ - $8y^{3}$ `vdots` 9
$\Rightarrow$ $x^{3}$ + $y^{3}$ - $9y^{3}$ `vdots` 9
Mà $-9y^{3}$ `vdots`9
$\Rightarrow$ $x^{3}$ + $y^{3}$ `vdots` 9 (đpcm)
