中山大学2020数学分析试题
证明:
设曲面为
$\displaystyle F(x,y,z)=0,$
假设有
$\displaystyle x=\phi (t),y=\psi (t),z=\omega (t),$
$\displaystyle t=t_0,(x_0,y_0,z_0)$
曲面在定点有连续偏导数,且$\displaystyle \phi'(t_0),\psi'(t_0),\omega'(t_0)$存在。则曲面在定点有全导数,且为零。即有
$\displaystyle \frac{\mathrm{d} }{\mathrm{d} t}F(x,y,z)|_{t=0}=0.$
得到
$\displaystyle F_x(x_0,y_0,z_0)\phi'(t_0)+F_y(x_0,y_0,z_0)\psi'(t_0)+F_z(x_0,y_0,z_0)\omega'(t_0)=0.$
定点的切向量为
$\displaystyle \overrightarrow{\tau}=(\phi'(t_0),\psi'(t_0),\omega'(t_0)),$
法向量与切向量垂直,由前式,即得到
$\displaystyle \overrightarrow{n}=\pm (F_x(x_0,y_0,z_0),F_y(x_0,y_0,z_0),F_z(x_0,y_0,z_0)).$