复合函数
f(x)=lnx
g(x)=ln[ln(x)]
r(x)=ln{lnln(x)]}
r'(x)=[1/lnln(x)]g'(x)=[1/lnln(x)][1/ln(x)]f'(x)=[1/lnln(x)][1/ln(x)](1/x)