对称轴为x=2f(x)=a(x-2)²+k则3=4a+k0=a+k所以a=1,k=-1f(x)=x²-4x+3ff(x)=(x-2)²-1-1<=x<=2在x=-2左边，递减f(-1)=8,f(2)=-1所以值域[-1,8]
