(3)
f(x)=log<a> [ (1-x)/(1+x) ]
f(-x) =log<a> [ (1+x)/(1-x) ]
=-log<a> [ (1-x)/(1+x) ]
=-f(x)
f 是奇函数
(4)
f(x) =log<a>[ x+√(x^2+1) ]
f(-x) =log<a>[ -x+√(x^2+1) ]
=log<a>[ {(x^2+1) - x^2}/{x+√(x^2+1) } ]
=log<a>[ 1/{x+√(x^2+1) } ]
= -log<a>[ x+√(x^2+1) ]
=-f(x)
f 是奇函数