△≥0
4a^2-4(a+6)≥0
a≥3,a≤-2
x1+x2=2a,x1x2=a+6
（x1-1）2+（x2-1）2
=(x1+x2)^2-2x1x2-2(x1+x2)+2
=4a^2-2(a+6)-4a+2
=4(a-3/4)^2-49/4
a≥3时,代数式≥8
a≤-2时,代数式≥18