a) x^2-2xy-4+y^2
= (x^2-2xy+y^2)-4
= (x-y)^2 - 2^2
= (x-y-2)(x-y+2)
b) x^2-y^2+5x+5y
= (x^2-y^2) + (5x+5y)
= (x-y)(x+y) + 5(x+y)
= (x+y)(x-y+5)
c) x^2+4xy-25+4y^2
= x^2+4xy-25+4y^2
= [x^2+4xy+4y^2] - 25
= [x^2+2.x.2y+(2y)^2] - 25
= (x+2y)^2 - 5^2
= (x+2y-5)(x+2y+5)
d) x^2+x-12
= (x^2+2.x.1/2+1/4) - 1/4 - 12
= (x+1/2)^2 - 49/4
= (x+1/2)^2 - (7/2)^2
= (x+1/2-7/2)(x+1/2+7/2)
= (x-3)(x+4)
Chúc bạn học tốt ^^