From the meaning of the question, we can get the slope of the straight line k BC = 1? 450? 23=? 3 10,

The slope k ′ = 0 of the straight line CA? 45 1? 23=- 125,

In order to make point c the only optimal solution of the objective function z=ax-y with the minimum value,

It is necessary to make the slope of the target straight line y a = ax-z between the slopes of the two straight lines,

Therefore, the range of a can be defined by (? 125,? 3 10),

So the answer is: (? 125,? 3 10)