N . F ik, i- , F i. :
F 12 + F 13 +...+ F 1 k +...+ F 1 N + F 1 =
F 21 + F 23 +...+ F 2 k +...+ F 2 N + F 2 =
..................................................................................
F i 1 + F i 2 +...+ F ik +...+ F iN + F i =
..................................................................................
F N 1 + F N 2 +...+ F Nk +...+ F N , N -1 + F N =
N . , F 12+ F 21=0 .., :
F 1 + F 2 +...+ F N =
, , :
,
d p / dt =0.
:
.
, , , 0, .
- 0, .