令y'=p,则y"=dp/dx=dp/dy *dy/dx=p*dp/dy.则方程化为p*dp/dy-9y=0即p*dp=9ydy,其通解为y'=p=根号(9y^2+c1),即dy/dx=根号(9y^2+c1）.也即dy/根号(9y^2+c1)=dx,解为x+c2=1/3 ln(y+根号(y^2+c1/9)) e^3(x+c2)=y+根号(y^...