6,14,18,19 giúp nhá mai mình ktra rồu!!!

6. $x^2+2x+1-y^2$
$=(x^2+2x+1)-y^2$

$=(x+1)^2-y^2$

$=(x+y+1)(x-y+1)$

14. $x^2-1+2xy+y^2$

$=(x^2+2xy+y^2)-1$

$=(x+y)^2-1$

$=(x+y+1)(x+y-1)$

18. $a^2-b^2-6b-9$

$=a^2-(b^2+6b+9)$

$=a^2-(b+3)^2$

$=(a+b+3)(a-b-3)$

19. $16x^2-y^2+6y-9$

$=16x^2-(y^2-6y+9)$

$=(4x)^2-(y-3)^2$

$=(4x+y-3)(4x-y+3)$.