ds v=lim △t-→0 -s ,D=D元0 = s = =) B ∴.i=v元0 心在直角坐标系中 0= d i+ dt dt 监=ui+u,j+uA OD月------------------------------------------------------------------------------- t s t   =  →0  lim s dt ds = =   = = = dt ds dt dr    0      = O A 1 r  B 3 r  C 2 r  0   0     =  在直角坐标系中  k i j k dt dz j dt dy i dt dx x y z         = + + = + +