y=x^2-ax+2=(x-a/2)^2+2-a^2/4 -1=a/2 a<-2 f(min)=f(-1)=3+a=-1 a=-2 1<=a/2 a>=2 f(min)=f(1)=3-a=-1 a=4 综上所述a=-2 a=4
