f(xy)=f(x)+f(y),x>0,y>0
当x>1,f(x)>0
x2>x1>0,x2/x1>1
f(x2)-f(x1)=f((x2/x1)*x1)-f(x1)
=f(x2/x1)+f(x1)-f(x1)
=f(x2/x1)>0
f(x)在（0,∞）上是增函数
f(1/3)=-1,f(1/9)=f(1/3)+f(1/3)=-2
f(x)-f(1/(x-2)≥2
f(x)-2≥f(1/(x-2))
f(9x)≥f(1/(x-2))
9x≥1/(x-2),x>0,x-2>0
9x^2-18x≥1,x>2
x≥（3+√10）/3