y=x^2+ax+a-3
x1+x2=-a
x1x2=a-3
(x1-x2)²
=(x1+x2)²-4x1x2
=a²-4(a-3)
=a²-4a+12
=a²-4a+4+8
=(a-2)²+8
最小距离为2√2