`(x^2+y^2-5)^2-4(x^2y^2-4xy+4)`
`=(x^2+y^2-5)^2-(4x^2y^2-16xy+16)`
`=(x^2+y^2-5)^2-[(2xy)^2-2.2xy.4+4^2]`
`=(x^2+y^2-5)^2-(2xy-4)^2`
`=[(x^2+y^2-5)-(2xy-4)][(x^2+y^2-5)+(2xy-4)]`
`=(x^2+y^2-5-2xy+4)(x^2+y^2-5+2xy-4)`
`=[(x^2-2xy+y^2)+(-5+4)][(x^2+2xy+y^2)+(-5-4)]`
`=[(x-y)^2-1][(x+y)^2-9]`
`=[(x-y)^2-1^2][(x+y)^2-3^2]`
`=(x-y-1)(x-y+1)(x+y-3)(x+y+3)`
$\text{@ThanhTruc2008}$
