f(x)=[m-g(x)]/[n+g(x)]
设g(x)=a^x
a^2=9,a=3
g(x)=3^x
f(x)=(m-3^x)/(n+3^x)
f(0)=(m-3^0)/(n+3^0)=0
m=1
f(x)=(1-3^x)/(n+3^x)
f(-x)=[1-3^(-x)]/[n+3^(-x)]
=(3^x-1)/(n3^x+1)
(1-3^x)/(n+3^x)=-(3^x-1)/(n3^x+1)
n+3^x=n3^x+1
(n-1)(3^x-1)=0
n=1
f(x)=(1-3^x)/(1+3^x)