设y=f(x),f(0)=0
f'(x)=2x+f(x)
设u=f(x)+2x,du/dx=f'(x)+2
du/dx -2 = f'(x)=2x+(u-2x)=u
du/(u+2)=dx
ln(u+2)=x+c
u=ce^x-2
f(x)=u-2x=ce^x-2x-2
f(0)=c-2=0,c=2
f(x)=2e^x-2x-2