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