y=3^x/(2+3^x)
2*y+y*3^x=3^x
3^x-y*3^x=2y
3^x(1-y)=2y
3^x=2y/(1-y)
lg3^x=x*lg3=lg[2y/(1-y)]
x=lg[2y/(1-y)]/lg3=log3 [2y/(1-y)]
所以反函数是
y=log3 [2x/(1-x)] 0