令g(x)=x^2-ax+3a由f(x)=1/根号(x^2-ax+3a)在区间[2,+无穷]上是减函数,须有g(x)=x^2-ax+3a在[2,+无穷大]为增函数,且大于零,则g(x)对称轴x=a/2<=2,解得a<=4g(2)>0,即4-2a+3a>0,解得a>-4所以-4{%<...