f(x)+2=a(x-1)(x-3),a<0 f(x)+6a=a(x-1)(x-3)+6a-2=ax^2-4ax+9a-2=0 相等实根->16a^2-4a(9a-2)=0 a=0.4 f(x)=0.4x^2-1.6x-0.8 x=2时f(x)最大，f(2)=-a-2>0 a<-2
