f(x) = ln（1+x) - ln(1-x)
= ln[(1+x)/(1-x)]
(1+x)/(1-x) > 0
(x+1)(x-1) < 0
-1< x< 1
定义域 :-1< x< 1
(2)
f(-x) = ln(1-x)/(1+x)
= -ln(1+x)/(1-x) = f(x)
f(x)为奇函数