Hence 
DIN: 
(ey era) 
Oz? Jpty da? dw 
dv? 
In the same way we find 
dey \2 
Ë ) Ol! Gey, 
Oy? /pTx dy” dw 
ve 
and 
dw dw 
(25) _ Pu _ de avaya 
da dy oT ow dy dw 
dv? 
So we can write (3) also under the following form: 
1 (dy | dw ae Hi aer 
laa Gate yal Ob ttt gated DO 
dv? 
or 
1 (Py a aa eat est =) ! ye 
dv? ze 
gL \2 
F 9% Ges 
EE 
022 
In order to satisfy this last equation for every arbitrary value of 
dv, dv and dy, the following equation must be satisfied: 
= ze 
dw 9 
Eet and > >0 and 3rd — 
The form sub. 3rd or 
(a dp & 
SIA gi Ee) ei A B | (dy 2m B IW)? 
(dx? gv? \ (dy 
2de? \dy du \ (Ox dy dv? de dvdy ba 
de Ov 
