1.(a-b)(-a-b)
= -(a-b)(a+b) = b^2 - a^2
2.(x-y)(y+x)^2
= (x-y)(y+x)*(x+y) = (x^2- y^2)(x+y) =x^3-y^3 +x^2y -xy^2
3.(x-y+z)^2
=[(x-y)+z]^2= (x-y)^2 + z^2 + 2z(x-y)
= x^2+y^2+2xy +z^2 +2xz-zyz
4.(x^2+y^2)(x^4+y^4)
=x^2*(x^4+y^4) + y^2*(x^4+y^4)
= x^6 +y^6 +x^2y^4 + y^2x^4