`a)x-xy+y-y^2`
`=x(1-y)+y(1-y)`
`=(1-y)(x+y)`
`b)x^2-4x-y^2+4`
`=-y^2-xy+2y+xy+x^2-2x-2y-2x+4`
`=-(y^2+xy-2y-xy-x^2+2x+2y+2x-4)`
`=-[y(y+x-2)-x(y+x-2)+2(y+x-2)]`
`=-(y-x+2)(y+x-2)`
`c)x^2-2x-3`
`=x^2+x-3x-3`
`=x(x+1)-3(x+1)`
`=(x+1)(x-3)`
`d)...+27`(không rõ đề