令a=(1+x)/x=1+1/x
1/x=a-1
x=1/(a-1)
2x/(1+2x)=[2/(a-1)]/[1+2/(a-1)]=2/[(a-1)+2]=2/(a+1)
所以f(a)=ln[2/(a+1)]
f(x)=ln[2/(x+1)]