8
根据韦达定理得：
x1+x2=2a
x1x2=a+6
(x1-1)^2+(x2-1)^2
=x1^2-2x1+1+x2^2-2x2+1
=(x1+x2)^2-2x1x2-2(x1+x2)+2
=4a^2-2(a+6)-2*2a+2
=4a^2-6a-10
=4(a-3/4)^2-12.25
判别式=4a^2-4(a+6)=4a^2-4a-24>=0
a^2-a-6>=0
(a-3)(a+2)>=0
a>=3或a