令t=x^2-ax+3,则该函数在(负无穷,0.5a]单调递减,要复合函数在(负无穷,0.5a]上单调递减,那么a>1
又x^2-ax+3在(负无穷,0.5a]上要恒大于0,那么
a^2/4-a^2/2+3>0
a^21
综上:1