方程改写为e^y(e^x+1)dy+e^x(e^y-1)dx=0,
分离变量,e^y/(e^y-1)dy=-e^x/(e^x+1)dx,
两边积分,ln(e^y-1)=-ln(e^x+1)+lnC.
(e^y-1)(e^x+1)=C