Bài 4:
1/  x^2-10x+25=(x-5)^2=(x-5)(x-5)
2/  x^2-64=(x-4)(x+4)
3/  25(x+y)^2-16(x-y)^2=(5(x+y)-4(x-y))(5(x+y)+4(x-y))=(5x+5y-4x+4y)(5x+5y+4x-4y)=(x+9y)(9x+y)
4/  x^4-1=(x^2-1)(x^2+1)=(x-1)(x+1)(x^2+1)
5/  2xy+3z+6y+xz=(2xy+6y)+(3z+xz)=2y(x+3)+z(3+x)=2y(x+3)+z(x+3)=(x+3)(2y+z)
6/  5x^2+5xy-x-y=5x(x+y)-(x+y)=(x+y)(5x-1)
Bài 5:
1/  x^2-2xy+y^2-xz+yz=(x-y)^2-z(x-y)=(x-y)(x-y-z)
2/  y-x^2y-2xy^2-y^3=-y(-1+x^2+2xy+y^2)=-y(x+y)^2-1=-y(x+y-1)(x+y+1)
3/  x^2-25+y^2+2xy=x^2+2xy+y^2-25=(x+y)^2-25=(x+y-5)(x+y+5)
4/  (x+y)^2-(x^2-y^2)=(x+y)^2-(x-y)(x+y)=(x+y)(x+y-x+y)=(x+y)2y
5/  x^2+4x-y^2+4=x^2+4x+4-y^2=(x+2)^2-y^2=(x+2-y)(x+2+y)
6/  2xy-x^2-y^2+16=-(x^2-2xy+y^2-16)=-(x-y)^2-16=-(x-y-4)(x-y+4)
Bài 6: 
1/  x^2+8x+7=x^2+x+7x+7=x(x+1)+7(x+1)=(x+1)(x+7)
2/  x^2-5x+6=x^2-2x-3x+6=x(x-2)-3(x-2)=(x-2)(x-3)
3/  x^2+3x-18=x^2+6x-3x-18=x(x+6)-3(x+6)=(x+6)(x-3)
4/  3x^2-16x+5=3x^2-x-15x+5=x(3x-1)-5(3x-1)=(3x-1)(x-5)