y=ln√(1+e^-2x)
=1/2 ln(1+e^-2x)
y''=1/2 [ln(1+e^-2x)]'
=1/2 * 1/(1+e^-2x)*(1+e^-2x)'
=1/[2(1+e^-2x)] *-2e^-2x
=-2e^-2x / [2(1+e^-2x)]
=-1/(e^2x+1)