1/ a*b*(x^2+1)+x*(a^2+b^2)
2/ x^2 - 2xy + y^2 - 4
= (x-y)^2 -2^2
= (x-y-2) (x-y+2)
3/ 9- x^2 - 2xy - y^2
= 9 - (x^2 + 2xy +y^2)
= 3^2 - (x+y)^2
= (3-x+y)(3+x+y)
4/ x^2 + y^2 + 2xy + yz + zx
= (x^2 + 2xy + y^2) + z(x+y)
= (x+y)^2 + z(x+y)
= (x+y+z)(x+y)