f(x)=1+2／（2^x-1）f(-x) = 1 + 2/(2^(-x)-1) = 1 - 2(2^x)/(2^x-1) = 1 - 2( (2^x-1)/(2^x-1)+1/(2^x-1)) = -1 - 2/(2^x-1) = -f(x)f 为奇函数
