Đáp án:
Giải thích các bước giải:
$a,x^2+4xy-21y^2$
$=x^2+4xy+4y^2-25y^2$
$=(x^2+4xy+4y)-(5y)^2$
$=(x+2y)^2-(5y)^2$
$=(x+2y-5y)(x+2y+5y)$
$=(x-3y)(x+7y)$
$b,5x^2+6xy+y^2$
$=5x^2+5xy+xy+y^2$
$=5x(x+y)+y(x+y)$
$=(5x+y)(x+y)$
$c,x^2+2xy-15y^2$
$=x^2+5xy-3xy-15y^2$
$=x(x+5y)-3y(x+5y)$
$=(x-3y)(x+5y)$
$d,(x-y)^2+4(x-y)-12$
$=(x-y)^2+2.(x-y).2+4-16$
$=[(x-y)^2+2.2(x-y)+4]-4^2$
$=(x-y+2)^2-4^2$
$=(x-y+2-4)(x-y+2+4)$
$=(x-y-2)(x-y+6)$
