x1+x2=2m
x1x2=a-m^2
x1^2+x2^2=(x1+x2)^2-2x1x2
=(2m)^2-2(a-m^2)
=4m^2-2a+2m^2
=6m^2-2a>=-2a
所以xi2+x22的最小值是-2a