2  Direction cosines of a line passing through two points
Since one and only one line passes through two given points, we can determine the
direction cosines of a line passing through the given points P(x1, y1, z1) and Q(x2, y2, z2)
as follows (Fig 11 3 (a)) Fig 11 3
r
Z
X
Y
R
S
P ( , , )
x y z
A
Oα
A
O
P
x
α
α
Fig 11