4a+2b=0
ax^2+bx=x
所以：ax^2+(b-1)x=0
x(ax+b-1)=0
∵有等根,而其中一个x = 0,
∴b=1
a=-1/2
f(x)=-x^2/2+x = -(x-1)^2 + 1/2
所以：值域为：（负无穷,1/2]