The difference between|△;and△r(△E (A≡△元) △: magnitude of△F Ar: the change of length B/△ of position vectors △ x2+y2+ Note △F≠△rThe difference between and ( ): r  r  r   r 1 r  P1 2 r  r P2   x y O r z 2 1 2 1 2 1 − x + y + z 2 2 2 2 2 2 r = x + y +z Note r : magnitude of   r   : the change of length of position vectors r ( r r )     r  