(2^x)+(2^-x)=5 两端同乘 2^x
(2^x)^2+1=5*2^x
(2^x)^2-5*(2^x)+1=0
2^x= [5±√21]/2 > 0
x=log(2)[5±√21]/2= log(2)[5±√21] -1
y=100/(1+2^-x)
1+2^(-x)=100/y
2^(-x)=100/y -1
-x=log(2) [100/y -1]
x=-log(2) [100/y -1]
∴ y=f^(-1) (x)= -log(2) [100/y -1]