1/2(x-a)-a=2-x
x=(4+3a)/3
(1)因为x为非正数所以x≤0即:(4+3a)/3≤0 a≤-1/3
(2)因为x不小于0.5所以x≥0.5即::(4+3a)/3≥0.5 a≥-5/6
-1/3≥a≥-5/6