Solution Since ar  is a unit vector, |
ra =| 1 Also,
(
) (
)
x
a
x
a
−
⋅
+
r
r
r
r  = 8
or
x x
x a
a x
a a
⋅
+
⋅
−
⋅
−
⋅
r r
r r
r r
r r  = 8
or
2
|
|
1
rx
 = 8  i e | rx |2 = 9
Therefore
|
|
xr  = 3 (as magnitude of a vector is non negative)