因为f(x) 是奇函数
所以 f(-x) = -f(x)
所以 f(x) + f(-x) = 0
1/(2^x - 1) + a + 1/[2^(-x) - 1] + a = 0
1/(2^x - 1) - 2^x/(2^x - 1) + 2a = 0
-1 + 2a = 0
a = 1/2