1.-a|-a|≥1,当a≥0,-a^2≥1舍,0>a,a≤-1
2.x≥a时,F(x)=2x^2+(x-a)^2=3x^2-2ax+a^2
对称轴x=a/3,当a>0时,最小值为2a^2
当a≤0时,最小值为2a^2/3
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