(x+y)(-x-y)=(x+y)(-1)(x+y)=(-1)(x+y) ^2=
=(-1)*(x^2+2xy+y^2)=-x^2-2xy-y^2
(-a-b)(-a-b) =(-1)(a+b)(-1)(a+b)=(a+b)^2=a^2+2ab+b^2
(x+y)(-x-y)=(x+y)(-1)(x+y)=(-1)(x+y) ^2=
=(-1)*(x^2+2xy+y^2)=-x^2-2xy-y^2
(-a-b)(-a-b) =(-1)(a+b)(-1)(a+b)=(a+b)^2=a^2+2ab+b^2