A
f(x)>=1
==>x^2-2ax+2a+3>=0
(x-a)^2+2a+3-a^2>=0
2a+3-a^2>=0
(a-1)^2-4<=0
所以 -1<=a<=3
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修改:dujs FROM 58.247.82.*
FROM 58.247.82.*