f[x]=3x^2-2ax+a^2,
(1)f[0]=a^2=1,a=土1.
(2)f(x)=3(x-a/3)^2+2a^2/3,
3a),
i)a>0,g(a)=2a^2-1>=0;或
ii)a0.
由i),a>=(√2)/2;
由ii),a