假设函数f(x)为奇函数
f(-x)=-f(x)
即:
a-2/(2^(-x)+1)=-a+2/(2^x+1)
2a=2/(2^(-x)+1)+2/(2^x+1)
a=1/(2^(-x)+1)+1/(2^x+1)
=2^x/(2^x+1)+1/(2^x+1)
=(2^x+1)/(2^x+1)
=1
a=1时函数f(x)为奇函数