Let P1
be the foot of the perpendicular from P on the plane XOY We, thus, see that P1 P is
parallel to z-axis As 
ˆ
ˆ ˆ
,  and 
i j
k  are the unit vectors along the x, y and z-axes,
respectively, and by the definition of the coordinates of P, we have 
1
ˆ
P P
OR
zk
=
=
uuur
uuur Similarly, 
1
ˆ
QP
OS
yj
=
=
uuur
uuur
 and 
ˆ
OQ
xi
=
uuur