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