f(x)=x^2+px+q
f(1)=1+p+q
f(2)=4+2p+q
f(3)=9+3p+q
f(1)+f(3)-2f(2)=1+p+q+9+3p+q-8-4p-2q=2
若f(1)|,|f(2)|,|f(3)|都小于1/2即
-1/2