令u=x+y则y=f(u)两边对x求导，得：y'=f'(u)*u'=f'(u)*(1+y')解得：y'=f'(u)/[1-f'(u)]故dy=f'(x+y)/[1-f'(x+y)]* dx
dy=df*(dx+dy)dy=df*dx/(1-df)
