21)
The four diagonals are OE, AF, BG and CD The direction cosines of the diagonal OE which
is the line joining two points O and E are
2
2
2
2
2
2
2
2
2
0
0
0
,
,
a
a
a
a
a
a
a
a
a
a
a
a
−
−
−
+ +
+ +
+ +
i e ,
3
1
, 
3
1
,  
3
1
B(0, , 0)
 a  
C(0, 0, )
a
a( , 0, )G
a
F(0, , )
a a
X
D( , , 0)
a
a
Y
Z
O
Fig 11