y = (2^x - 1) / (2^x + 1)
即：
f(x) = (2^x - 1) / (2^x + 1)
所以：
f(-x) = [2^(-x) - 1] / [2^(-x) + 1]
= [1/(2^x) - 1] / [1/(2^x) + 1]【分子分母同时乘以(2^x)】
= [1 - (2^x)] / [1 + (2^x)]
= - (2^x - 1) / (2^x + 1)
= - f(x)
即
f(-x) = - f(x)
所以函数为奇函数.