x & gt=a，f (x) = ( 1-a) x-a。

x & lta，f (x) =-( 1+a) x+a。

From the title, we know that x & gt=a, f(x) rises monotonously, and X.

So1-a >; =0，-( 1+a)& lt； =0, and a>0,

So 0