y=(e^x-e^-x)/(e^x+e^-x) =(e^2x-1)/(e^2x+1),(分子分母同乘：e^x)
=[(e^2x+1)-2]/(e^2x+1)
=1-2/(e^2x+1)
y'=-2*(-1)*1/(e^2x+1)^2*(e^2x+1)'
=2/(e^2x+1)^2*e^2x*(2x)'
=4e^2x/(e^2x+1)^2