若判别式小于等于0,则恒大于等于0
(a+3)^2-12a<=0
(a-6)^2<=0
则只有a=6
a>6,判别式大于0
x^2-(a+3)x+3a=0的根是x=[(a+3)±(a-6)]/2
x=(2a-3)/2,x=9/2
a>6,则(2a-3)/2>9/2
a<6,(2a-3)/2<9/2
综上
a>6,9/2
a<6,(2a-3)/2
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