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