dy/dx=e^(x-y-2)
dy/dx=e^-2*e^x/e^-y
e^ydy=e^-2e^xdx
两边积分得到
e^y=e^-2e^x+C
代入 （0,0）
1=e^-2+C=0
e^y=e^-2e^x+1-e^-2