令b=2^x,则b=3
[2^(3x)-2(-3x)]/[(2^x)-2^(-x)]
=(b^3-b^-3)/(b-b^-1)
=(b-b^-1)*(b^2+1+b^-2)/(b-b^-1)
=b^2+1+b^-2
=9+1+1/9
=91/9