f(2)=4a+2b=0
f(x)-x=ax^2+(b-1)x=0
(b-1)^2=0
b=1
a=-1/2
f(x)=-1/2x^2+x
=-1/2(x-1)^2+1/2在[1,2]上单减
f(x)∈[f(2),f(1)]=[0,1/2]