Let OADBFEGC be the cube with each side of length a units (Fig 11 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