带入x=1
ax^2=1
x-b=1-b
a=1-b
带入x=4/3
4/3-b=1
b=1/3
a=1-b=2/3
2)f(x)=0 (-无穷,0]
=2ax=4x/3 (0,1]
=1 (1,4.3]
=0 (4/3,无穷)