a/ $x²+4y²-4xy+3x-6y-4$
$=(x²-4xy+4y²)+3(x-2y)-4$
$=(x-2y)²+4(x-2y)-(x-2y)-4$
$=(x-2y)(x-2y+4)-[(x-2y)+4]$
$=(x-2y+4)(x-2y-1)$
b/ $(x²+8x+7)(x+3)(x+5)+15$
$=(x²+8x+7)[(x+3)(x+5)]+15$
$=(x²+8x+7)(x²+8x+15)+15$
Đặt $x²+8x+11=t$
$→(t-4)(t+4)+15$
$=t²-16+15$
$=t²-1$
$=(t-1)(t+1)$
$=(x²+8x+11-1)(x²+8x+11+1)$
$=(x²+8x+10)(x²+8x+12)$