Solution Let r
x i
y j
∧
∧
=
+
r
 be a unit vector in XY-plane (Fig 10 28) Then, from the
figure, we have x = cos θ and y = sin θ (since | rr | = 1) So, we may write the vector rr as
(
OP)
r =
uuur
r
=
ˆ
ˆ
cos
sin
i
j