有两根
4a^2-4(a+6)>=0
a^2-a-6>=0
a>=3,a<=-2
x+y=2a
xy=a+6
(x+y)^2=4a^2
x^2+y^2=4a^2-2xy=4a^2-2a-12
(x-1)^2+(y-1)^2
=(x^2+y^2)-2(x+y)+2
=4a^2-2a-12-4a+2
=4a^2-6a-10
=4(a-3/4)^2-49/4
a>=3,a<=-2
所以a=3时
最小值=8