|ax-1|≥x
|ax-1|^2≥x2
(a-1)x2-2ax+1≥0
当a=1,x≤1/2
当a≠1
x1=a±√(4(a-1/2)^2+3)/2
a>1时,x≤a-√(4(a-1/2)^2+3)/2或x≥a+√(4(a-1/2)^2+3)/2
a