将fx转换：f(x) = lg((1 - x) / (1 + x)) = lg(1-x) - lg(1 + x)
则f(-x) = lg(1-(-x)) - lg(1 + (-x)) = lg(1 + x) - lg(1 - x)
= -(lg(1 -x) - lg(1+ x)) = -f(x)
所以f(-a) = - f(a) = -b