2dy/(y^2-1)=dx
左边等于{[1/(y-1)]-[1/(y+1)]}dy
两边积分
得到ln(y-1)-ln(y+1)=x
(y-1)/(y+1)=e^x
y-1=(e^x)y+e^x
y=(e^x+1)/(1-e^x)