3^x：表示3的x次方
y=[3^x]/[(3^x)＋1]
y[(3^x)＋1]=3^x
y×(3^x)＋y=3^x
(3^x)－y(3^x)=y
(1－y)(3^x)=y
3^x=y/(1－y)
x=log(2)[(y)/(1－y)]
则反函数是：y=log(3)[(x)/(1－x)]