f(x)=lg1-x/1+x,且f(x)+f(y)=f(z),z=
问题描述：
f(x)=lg1-x/1+x,且f(x)+f(y)=f(z),z=
答
f(x)+f(y)= lg (1-x)/(1+x) + lg (1-y)/(1+y) = lg (1-x)(1-y)/(1+x)(1+y)
= lg (1 + xy - x - y)/(1 + xy + x + y)
= lg [ 1 - (x+y)/(1+xy) ]/[ 1 + (x+y)/(1+xy) ]
= f( (x+y)/(1+xy) )
所以 z= (x+y)/(1+xy)