y=(e^2x)/(e^x+1)
所以
y'=[(e^2x)'(e^x+1)-(e^2x)(e^x+1)']/(e^x+1)^2
=[2e^2x(e^x+1)-(e^2x)(e^x)]/(e^x+1)^2
=[e^2x(2e^x+2-e^x)]/(e^x+1)^2
=[e^2x(e^x+2)]/(e^x+1)^2
所以
y''=[(e^3x+2e^2x)'[e^x+1]^2-(e^3x+2e^2x)[[e^x+1]^2]']/(e^x+1)^4
=[(3e^3x+4e^2x)[e^x+1]^2 - (e^3x+2e^2x)[2e^x+1](e^x)]/(e^x+1)^4