p:partial (偏微)    del=(p/px)Xdirection+(p/py)Ydirection+(p/pz)Zdirection     gradient V=(pV/px)Xdirection+(pV/py)Ydirection+(pV/pz)Zdirection    vector                       divergence V=pV/px)+(pV/py)+(pV/pz )                                                         scalar   curl V=determinant   Xdirection,Ydirection,Zdirection    p/px,           p/py,           p/pz   Ax,               Ay ,           Az     方向導數=gradient v *單位向量