对于任意x1>x2≥3,有f(x1)>f(x2)
所以x1+2a/x1-x2-2a/x2>0
(x1-x2)-2a(x1-x2)/x1x2>0
(x1-x2)(1-2a/x1x2)>0 因为x1-x2>0所以
1-2a/x1x2>0
ax2≥3,即x1x2/2>9/2
所以a≤9/2