1)1-x^2-2xy-y^2
=1-(x^2+2xy+y^2)
=1-(x+y)^2
=(1+x+y)(1-x-y)
(2)x^2+4ax+4a^2-y^2
=(x+2a)^2-y^2
=(x+2a+y)(x+2a-y)