F(x)=x^2-ax-3的对称轴是x=a/2,F(x)在(-无穷大,2)上单调,则a/2>=2 a>=4
F(1)F(2)=(-2-a)(1-2a)=2a^2+3a-2(a>=4)对称轴是a=-3/4
F(1)F(2)的值域是[-27/8,+无穷)
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